Development_Setup.rst: Add Python 3 consistently
Development_Setup.rst: Add Python 3 consistently

Change all references of pip and python
to pip3 and python3. Also add a section explaining
what to do when different versions of Python were installed.

Closes https://github.com/coala/coala/issues/3921

refeed/coala,Asalle/coala,arush0311/coala,rimacone/testing2,SanketDG/coala,Shade5/coala,coala/coala,MariosPanag/coala,Asnelchristian/coala,yashLadha/coala,coala/coala,coala-analyzer/coala,refeed/coala,Nosferatul/coala,Shade5/coala,RJ722/coala,coala/coala,Nosferatul/coala,jayvdb/coala,MariosPanag/coala,jayvdb/coala,JohnS-01/coala,netman92/coala,karansingh1559/coala,CruiseDevice/coala,nemaniarjun/coala,netman92/coala,kartikeys98/coala,shreyans800755/coala,shreyans800755/coala,SanketDG/coala,Asalle/coala,CruiseDevice/coala,nemaniarjun/coala,aptrishu/coala,SanketDG/coala,Balaji2198/coala,damngamerz/coala,kartikeys98/coala,coala-analyzer/coala,arjunsinghy96/coala,yashLadha/coala,incorrectusername/coala,Nosferatul/coala,jayvdb/coala,karansingh1559/coala,Shade5/coala,kartikeys98/coala,rimacone/testing2,MariosPanag/coala,Asnelchristian/coala,RJ722/coala,refeed/coala,ayushin78/coala,karansingh1559/coala,JohnS-01/coala,Balaji2198/coala,Balaji2198/coala,arush0311/coala,Asalle/coala,aptrishu/coala,CruiseDevice/coala,incorrectusername/coala,RJ722/coala,arjunsinghy96/coala,netman92/coala,damngamerz/coala,nemaniarjun/coala,damngamerz/coala,ayushin78/coala,incorrectusername/coala,rimacone/testing2,ayushin78/coala,arjunsinghy96/coala,JohnS-01/coala,Asnelchristian/coala,coala-analyzer/coala,arush0311/coala,yashLadha/coala,aptrishu/coala,shreyans800755/coala