Remove Python 2 conditional code

[kurtmckee]

This PR introduces the following changes:

• Remove Python 2 conditional code
• Add a tox pylint environment that mirrors the CI configuration.

Note

No Python 3 / C compatibility functions were modified yet, but these need to be removed as well.

These can be found later by searching the codebase for PY_MAJOR_VERSION and PY_VERSION_HEX references.

[coveralls]

coverage: 28.845% (+0.1%) from 28.747%
when pulling 893f13c on kurtmckee:rm-python-2-code
into 8f3331c on LudovicRousseau:master.

[LudovicRousseau]

Thanks