C Extension not building on Windows for cp27

[mayeut]

This results in no speed-ups compared to the stock Python implementation.

Root cause:
libbase64 requires a C99 compiler.
Windows CPython 2.7 requires to build with Visual C++ 9.0 that is not compliant.
Refer to https://wiki.python.org/moin/WindowsCompilers.

Possible solutions:

1. Propose PR to upstream libbase64 to have support for those compilers. => unlikely to be merged.
2. Fork libbase64 to have support for those compilers. => I'd rather not.
3. Do nothing about it.

[mayeut]

Let's drop python 2.7 support: https://python3statement.org