Define _PyUnicode_DecodeUnicodeEscape even on Python 3.6+

[hroncok]

Fixes #169

I have tested this on 3.9.8, 3.9.7 and 3.10.0 only. The CI tested this more.

[hroncok]

Some OSes on the CI install 3.9.7 and some install 3.9.8, which is convenient. It passes on both.

[hauntsaninja]

Awesome, thank you!

[srittau]

My knowledge of Python internals and typed_ast is very limited, but wouldn't it be safer to get rid of _PyUnicode_DecodeUnicodeEscape and call PyUnicode_DecodeUnicodeEscape directly? All the underlined function does is set first_invalid_escape of the only caller to NULL.

Edit: And that would mean the following block never gets called.

[hroncok]

My knowledge of typed_ast is nonexistent. I have no idea. I think this is a shim needed to support pre-3.6 Python as well.

[srittau]

Well, let's merge this. I will send a separate PR trying my idea.

[hauntsaninja]

So the case where we hit that block previously is, for example:

from typed_ast import ast3
ast3.parse(r'''
print('\Q')
''')

The only user visible difference I've been able to see is that if running with e.g. python -Werror, a SyntaxError is raised instead of a DeprecationWarning. The reason the code does this translation is because SyntaxErrors provide location information. See warn_invalid_escape_sequence. The resulting slight difference in behaviour between Python versions seems fine to me / seemed fine to whoever implemented this for Python 3.5 as well.