Make CallbackStream compatible with Python 3.6
Make CallbackStream compatible with Python 3.6

Summary:
Adding the attributes listed enables using pdb() inside
python_unittest targets.

Before:

    $ buck-out/gen/tests\#binary.par tests.TypeCoverageTests.test_simple_module
    test_simple_module (tests.TypeCoverageTests) ... > buck-out/dev/gen/tests#binary,link-tree/type_coverage.py(64)_g_module()
    -> if hasattr(node, 'body'):
    ERROR

    ======================================================================
    ERROR: test_simple_module (tests.TypeCoverageTests)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
    ...
    AttributeError: 'CallbackStream' object has no attribute 'errors'

    ----------------------------------------------------------------------
    Ran 1 test in 2.177s

    FAILED (errors=1)

After:

    $ buck-out/gen/tests\#binary.par tests.TypeCoverageTests.test_simple_module
    test_simple_module (tests.TypeCoverageTests) ... > buck-out/dev/gen/tests#binary,link-tree/type_coverage.py(64)_g_module()
    -> if hasattr(node, 'body'):
    (Pdb) c
    ok

    ----------------------------------------------------------------------
    Ran 1 test in 1.097s

    OK

Test Plan: As described in the summary.

Reviewed By: andrewjcg

fbshipit-source-id: 3e1b35a

clonetwin26/buck,robbertvanginkel/buck,vschs007/buck,JoelMarcey/buck,zhan-xiong/buck,dsyang/buck,Addepar/buck,shs96c/buck,shs96c/buck,k21/buck,kageiit/buck,LegNeato/buck,rmaz/buck,k21/buck,marcinkwiatkowski/buck,zpao/buck,nguyentruongtho/buck,robbertvanginkel/buck,zhan-xiong/buck,kageiit/buck,shybovycha/buck,zhan-xiong/buck,nguyentruongtho/buck,marcinkwiatkowski/buck,vschs007/buck,shs96c/buck,marcinkwiatkowski/buck,brettwooldridge/buck,ilya-klyuchnikov/buck,robbertvanginkel/buck,clonetwin26/buck,Addepar/buck,Addepar/buck,romanoid/buck,clonetwin26/buck,robbertvanginkel/buck,robbertvanginkel/buck,JoelMarcey/buck,marcinkwiatkowski/buck,clonetwin26/buck,vschs007/buck,kageiit/buck,dsyang/buck,clonetwin26/buck,shybovycha/buck,zpao/buck,brettwooldridge/buck,k21/buck,marcinkwiatkowski/buck,dsyang/buck,facebook/buck,robbertvanginkel/buck,k21/buck,shybovycha/buck,shybovycha/buck,ilya-klyuchnikov/buck,k21/buck,zhan-xiong/buck,LegNeato/buck,vschs007/buck,zhan-xiong/buck,robbertvanginkel/buck,zhan-xiong/buck,shybovycha/buck,vschs007/buck,rmaz/buck,vschs007/buck,robbertvanginkel/buck,Addepar/buck,k21/buck,ilya-klyuchnikov/buck,JoelMarcey/buck,SeleniumHQ/buck,zpao/buck,clonetwin26/buck,JoelMarcey/buck,dsyang/buck,shybovycha/buck,romanoid/buck,Addepar/buck,zpao/buck,zpao/buck,Addepar/buck,shs96c/buck,ilya-klyuchnikov/buck,shybovycha/buck,shybovycha/buck,vschs007/buck,vschs007/buck,marcinkwiatkowski/buck,SeleniumHQ/buck,SeleniumHQ/buck,nguyentruongtho/buck,shs96c/buck,SeleniumHQ/buck,zhan-xiong/buck,kageiit/buck,vschs007/buck,romanoid/buck,nguyentruongtho/buck,k21/buck,ilya-klyuchnikov/buck,SeleniumHQ/buck,robbertvanginkel/buck,facebook/buck,robbertvanginkel/buck,LegNeato/buck,dsyang/buck,JoelMarcey/buck,shs96c/buck,zpao/buck,dsyang/buck,Addepar/buck,facebook/buck,Addepar/buck,brettwooldridge/buck,shybovycha/buck,LegNeato/buck,LegNeato/buck,facebook/buck,nguyentruongtho/buck,SeleniumHQ/buck,LegNeato/buck,romanoid/buck,ilya-klyuchnikov/buck,shybovycha/buck,zhan-xiong/buck,kageiit/buck,LegNeato/buck,romanoid/buck,shybovycha/buck,shybovycha/buck,robbertvanginkel/buck,JoelMarcey/buck,k21/buck,Addepar/buck,vschs007/buck,zhan-xiong/buck,dsyang/buck,k21/buck,LegNeato/buck,zhan-xiong/buck,rmaz/buck,k21/buck,brettwooldridge/buck,brettwooldridge/buck,marcinkwiatkowski/buck,LegNeato/buck,JoelMarcey/buck,JoelMarcey/buck,marcinkwiatkowski/buck,dsyang/buck,shs96c/buck,rmaz/buck,brettwooldridge/buck,LegNeato/buck,SeleniumHQ/buck,nguyentruongtho/buck,romanoid/buck,dsyang/buck,JoelMarcey/buck,brettwooldridge/buck,kageiit/buck,romanoid/buck,JoelMarcey/buck,shs96c/buck,ilya-klyuchnikov/buck,shybovycha/buck,Addepar/buck,clonetwin26/buck,shs96c/buck,marcinkwiatkowski/buck,shs96c/buck,dsyang/buck,LegNeato/buck,Addepar/buck,SeleniumHQ/buck,dsyang/buck,brettwooldridge/buck,clonetwin26/buck,rmaz/buck,clonetwin26/buck,JoelMarcey/buck,zhan-xiong/buck,romanoid/buck,SeleniumHQ/buck,nguyentruongtho/buck,SeleniumHQ/buck,clonetwin26/buck,robbertvanginkel/buck,romanoid/buck,JoelMarcey/buck,shs96c/buck,clonetwin26/buck,brettwooldridge/buck,LegNeato/buck,SeleniumHQ/buck,ilya-klyuchnikov/buck,rmaz/buck,zpao/buck,brettwooldridge/buck,shs96c/buck,zhan-xiong/buck,JoelMarcey/buck,romanoid/buck,Addepar/buck,vschs007/buck,romanoid/buck,kageiit/buck,SeleniumHQ/buck,brettwooldridge/buck,shs96c/buck,ilya-klyuchnikov/buck,marcinkwiatkowski/buck,rmaz/buck,brettwooldridge/buck,zhan-xiong/buck,rmaz/buck,robbertvanginkel/buck,brettwooldridge/buck,marcinkwiatkowski/buck,romanoid/buck,k21/buck,vschs007/buck,dsyang/buck,marcinkwiatkowski/buck,rmaz/buck,clonetwin26/buck,LegNeato/buck,Addepar/buck,romanoid/buck,rmaz/buck,facebook/buck,facebook/buck,SeleniumHQ/buck,facebook/buck,ilya-klyuchnikov/buck,vschs007/buck,rmaz/buck,dsyang/buck,rmaz/buck,clonetwin26/buck,k21/buck,rmaz/buck,k21/buck,marcinkwiatkowski/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck