[SPARK-16268][PYSPARK] SQLContext should import DataStreamReader
[SPARK-16268][PYSPARK] SQLContext should import DataStreamReader

## What changes were proposed in this pull request?

Fixed the following error:
```
>>> sqlContext.readStream
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "...", line 442, in readStream
    return DataStreamReader(self._wrapped)
NameError: global name 'DataStreamReader' is not defined
```

## How was this patch tested?

The added test.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #13958 from zsxwing/fix-import.

ptkool/spark,UndeadBaneGitHub/spark,dongjoon-hyun/spark,vinodkc/spark,srowen/spark,liyichao/spark,ajaysaini725/spark,shubhamchopra/spark,SnappyDataInc/spark,bravo-zhang/spark,ahnqirage/spark,byakuinss/spark,rednaxelafx/apache-spark,zuotingbing/spark,milliman/spark,techaddict/spark,kevinyu98/spark,michalsenkyr/spark,bdrillard/spark,eyalfa/spark,tengpeng/spark,jkbradley/spark,tengpeng/spark,taroplus/spark,aray/spark,nlalevee/spark,yanboliang/spark,srowen/spark,techaddict/spark,minixalpha/spark,HyukjinKwon/spark,liyichao/spark,Aegeaner/spark,rednaxelafx/apache-spark,aokolnychyi/spark,zuotingbing/spark,darionyaphet/spark,mdespriee/spark,map222/spark,kiszk/spark,zero323/spark,ptkool/spark,someorz/spark,vinodkc/spark,facaiy/spark,narahari92/spark,liutang123/spark,1haodian/spark,1haodian/spark,aosagie/spark,nlalevee/spark,shaneknapp/spark,cin/spark,rednaxelafx/apache-spark,szhem/spark,highfei2011/spark,mike0sv/spark,shuangshuangwang/spark,rekhajoshm/spark,janewangfb/spark,WeichenXu123/spark,hvanhovell/spark,ron8hu/spark,dongjoon-hyun/spark,BryanCutler/spark,wzhfy/spark,MLnick/spark,jkbradley/spark,xuanyuanking/spark,zuotingbing/spark,ueshin/apache-spark,zhouyejoe/spark,publicRoman/spark,szhem/spark,michalsenkyr/spark,loneknightpy/spark,ajaysaini725/spark,taroplus/spark,ajaysaini725/spark,xuanyuanking/spark,publicRoman/spark,icexelloss/spark,xflin/spark,debugger87/spark,chuckchen/spark,map222/spark,gengliangwang/spark,wangyum/spark,metamx/spark,ioana-delaney/spark,pgandhi999/spark,joseph-torres/spark,liutang123/spark,rikima/spark,vinodkc/spark,shaneknapp/spark,debugger87/spark,HyukjinKwon/spark,eyalfa/spark,saltstar/spark,cloud-fan/spark,liutang123/spark,cloud-fan/spark,zero323/spark,bravo-zhang/spark,ddna1021/spark,guoxiaolongzte/spark,MLnick/spark,setjet/spark,shubhamchopra/spark,wzhfy/spark,szhem/spark,nilsgrabbert/spark,janewangfb/spark,lxsmnv/spark,esi-mineset/spark,kissmetrics/spark,tengpeng/spark,wzhfy/spark,srowen/spark,byakuinss/spark,ahnqirage/spark,bOOm-X/spark,adrian-ionescu/apache-spark,alunarbeach/spark,apache-spark-on-k8s/spark,WeichenXu123/spark,wangmiao1981/spark,zhouyejoe/spark,holdenk/spark,WeichenXu123/spark,mike0sv/spark,akopich/spark,wzhfy/spark,jrshust/spark,facaiy/spark,dhruve/spark,jlopezmalla/spark,WindCanDie/spark,wangyum/spark,aray/spark,jianran/spark,rednaxelafx/apache-spark,ddna1021/spark,Panos-Bletsos/spark-cost-model-optimizer,milliman/spark,ibm-research-ireland/sparkoscope,stanzhai/spark,kevinyu98/spark,ueshin/apache-spark,publicRoman/spark,shuangshuangwang/spark,kevinyu98/spark,kiszk/spark,saturday-shi/spark,ron8hu/spark,zhouyejoe/spark,ibm-research-ireland/sparkoscope,nilsgrabbert/spark,markhamstra/spark,spark-test/spark,ron8hu/spark,icexelloss/spark,nlalevee/spark,shuangshuangwang/spark,poffuomo/spark,bravo-zhang/spark,WindCanDie/spark,liyichao/spark,bdrillard/spark,highfei2011/spark,hhbyyh/spark,caneGuy/spark,WindCanDie/spark,apache-spark-on-k8s/spark,dongjoon-hyun/spark,gengliangwang/spark,skonto/spark,skonto/spark,Aegeaner/spark,mzl9039/spark,apache/spark,techaddict/spark,kevinyu98/spark,alunarbeach/spark,ptkool/spark,eyalfa/spark,nchammas/spark,sahilTakiar/spark,hvanhovell/spark,cin/spark,andrewor14/spark,adrian-ionescu/apache-spark,dongjoon-hyun/spark,actuaryzhang/spark,vinodkc/spark,sureshthalamati/spark,SnappyDataInc/spark,shaneknapp/spark,jianran/spark,ddna1021/spark,panfengfeng/spark,rezasafi/spark,maropu/spark,jrshust/spark,janewangfb/spark,big-pegasus/spark,actuaryzhang/spark,spark-test/spark,cloud-fan/spark,joseph-torres/spark,markhamstra/spark,bOOm-X/spark,patrick-nicholson/spark,ron8hu/spark,JoshRosen/spark,witgo/spark,mzl9039/spark,Panos-Bletsos/spark-cost-model-optimizer,skonto/spark,mdespriee/spark,MLnick/spark,saltstar/spark,xflin/spark,techaddict/spark,maropu/spark,loneknightpy/spark,JerryLead/spark,ioana-delaney/spark,esi-mineset/spark,chuckchen/spark,tejasapatil/spark,rezasafi/spark,guoxiaolongzte/spark,maropu/spark,janewangfb/spark,LantaoJin/spark,akopich/spark,witgo/spark,narahari92/spark,nilsgrabbert/spark,hvanhovell/spark,wangmiao1981/spark,LantaoJin/spark,spark-test/spark,shubhamchopra/spark,setjet/spark,SHASHANKB/spark,brad-kaiser/spark,big-pegasus/spark,bdrillard/spark,debugger87/spark,patrick-nicholson/spark,mzl9039/spark,wzhfy/spark,mdespriee/spark,Panos-Bletsos/spark-cost-model-optimizer,sureshthalamati/spark,mzl9039/spark,big-pegasus/spark,jkbradley/spark,hhbyyh/spark,esi-mineset/spark,dbtsai/spark,JerryLead/spark,rikima/spark,BryanCutler/spark,wzhfy/spark,dongjoon-hyun/spark,guoxiaolongzte/spark,JerryLead/spark,hhbyyh/spark,stanzhai/spark,ConeyLiu/spark,panfengfeng/spark,maropu/spark,gengliangwang/spark,hvanhovell/spark,highfei2011/spark,JerryLead/spark,Aegeaner/spark,bravo-zhang/spark,goldmedal/spark,ajaysaini725/spark,sureshthalamati/spark,kimoonkim/spark,ConeyLiu/spark,eyalfa/spark,rekhajoshm/spark,SHASHANKB/spark,nchammas/spark,publicRoman/spark,rezasafi/spark,ahnqirage/spark,big-pegasus/spark,zzcclp/spark,akopich/spark,wangmiao1981/spark,spar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