Clarify behaviour `cStringIO.StringIO` behaviour wrt `unicode`

[moreati]

I thought cStringIO.StringIO was stricter about unicode objects than StringIO.StringIO and that explained 34751c3. However I can't reproduce that difference now.

[moreati]

That code should probably prefer io module, rather than fallback to it.

[moreati]

https://gist.github.com/moreati/6e0239e9057e8fda147ce12321011e5d

➜  mitogen git:(issue1265-experiment) ✗ python2 memio.py 
Python  Constructor                 Result                           
------  --------------------------  ---------------------------------
py2.7   io.BytesIO()                <_io.BytesIO object ...>         
py2.7   io.BytesIO(b'abc')          <_io.BytesIO object ...>         
py2.7   io.BytesIO(u'abc')          TypeError                        
py2.7   io.BytesIO(u'αβγ')          TypeError                        
py2.7   io.StringIO()               <_io.StringIO object ...>        
py2.7   io.StringIO(b'abc')         TypeError                        
py2.7   io.StringIO(u'abc')         <_io.StringIO object ...>        
py2.7   io.StringIO(u'αβγ')         <_io.StringIO object ...>        
py2.7   cStringIO.StringIO()        <cStringIO.StringO object ...>   
py2.7   cStringIO.StringIO(b'abc')  <cStringIO.StringI object ...>   
py2.7   cStringIO.StringIO(u'abc')  <cStringIO.StringI object ...>   
py2.7   cStringIO.StringIO(u'αβγ')  UnicodeEncodeError               
py2.7   StringIO.StringIO()         <StringIO.StringIO instance ...> 
py2.7   StringIO.StringIO(b'abc')   <StringIO.StringIO instance ...> 
py2.7   StringIO.StringIO(u'abc')   <StringIO.StringIO instance ...> 
py2.7   StringIO.StringIO(u'αβγ')   <StringIO.StringIO instance ...> 

Python  Constructor                                                          Result    
------  -------------------------------------------------------------------  ----------
py2.7   StringIO.StringIO().getvalue()                                       b''       
py2.7   StringIO.StringIO(b'a').getvalue()                                   b'a'      
py2.7   StringIO.StringIO(u'a').getvalue()                                   u'a'      
py2.7   s = StringIO.StringIO(u'\u03b1\u03b2\u03b3'); s.getvalue()           u'αβγ'    
py2.7   s = StringIO.StringIO(); s.write(b'a'); s.getvalue()                 b'a'      
py2.7   s = StringIO.StringIO(); s.write(u'a'); s.getvalue()                 u'a'      
py2.7   s = StringIO.StringIO(); s.write(b'a'); s.write(u'a'); s.getvalue()  u'aa'     
py2.7   s = StringIO.StringIO(); s.write(u'a'); s.write(b'a'); s.getvalue()  u'aa'     

➜  mitogen git:(issue1265-experiment) ✗ python3 memio.py 
Python  Constructor                 Result                           
------  --------------------------  ---------------------------------
py3.9   io.BytesIO()                <_io.BytesIO object ...>         
py3.9   io.BytesIO(b'abc')          <_io.BytesIO object ...>         
py3.9   io.BytesIO(u'abc')          TypeError                        
py3.9   io.BytesIO(u'αβγ')          TypeError                        
py3.9   io.StringIO()               <_io.StringIO object ...>        
py3.9   io.StringIO(b'abc')         TypeError                        
py3.9   io.StringIO(u'abc')         <_io.StringIO object ...>        
py3.9   io.StringIO(u'αβγ')         <_io.StringIO object ...>        

[moreati]

Updated https://github.com/mitogen-hq/mitogen/wiki/Python-2.x-3.x-compatibility-notes#in-memory-io