Decorator

The decorator module was not copying the __module__ attribute anymore.
Thanks to Nikolay Markov for the notice.

Dropped support for Python < 3.5 with a substantial simplification of
the code base (now building a decorator does not require calling "exec").
Added a way to mimic functools.wraps-generated decorators.
Ported the Continuous Integration from Travis to GitHub.

Sylvan Mosberger (https://github.com/Infinisil) contributed a patch to
some doctests that were breaking on NixOS.
John Vandenberg (https://github.com/jayvdb) made a case for removing the usage
of `__file__`, that was breaking PyOxidizer.
Miro Hrončok (https://github.com/hroncok) contributed some fixes for the
future Python 3.9.
Hugo van Kemenade (https://github.com/hugovk) contributed some fixes for the
future Python 3.10.

Changed the description to "Decorators for Humans" are requested by
several users. Fixed a .rst bug in the description as seen in PyPI.

Fixed a regression with decorator factories breaking the case with no
arguments by going back to the syntax used in version 4.2.
Accepted a small fix from Eric Larson (https://github.com/larsoner) affecting
`isgeneratorfunction` for old Python versions.
Moved the documentation from ReadTheDocs to GitHub to simplify the
release process and replaced ReStructuredText with Markdown: it is
an inferior solution, but it works better with GitHub and it is good enough.

Accepted a patch from Sylvain Marie (https://github.com/smarie): now the
decorator module can decorate generator functions by preserving their
being generator functions. Set `python_requires='>=2.6, !=3.0.*, !=3.1.*'`
in setup.py, as suggested by https://github.com/hugovk.