One can now use either the $CU2QU_WITH_CYTHON environment variable
or the --with/without-cython command line options to setup.py.
The --without-cython is for when one wants to get a pure, universal
wheel despite having both cython and a C compiler installed.
By default (when no env var nor command line options are used),
building from source will attempt to compile the extension module
using the pre-generated *.c source files that are always included
in the sdist package that we (will) upload to PyPI.
If the compilation fails (e.g. the user doesn't have a C compiler),
then an error message will be printed but installation will proceed
without the optional native extension.
When cloning a fresh repo, the *.c sources will not be present, so
to build the extension from local checkout one always needs Cython
installed.
When $CU2QU_WITH_CYTHON is true (1, or yes), then cython is required
and installation will abort if any error occurs during building.
when self.sign_tags is False, we were passing '--sign-tags False' to bumpversion script.
The boolean option with False value should be omitted instead.
This has been incorrect since the first PyPI release.
It was probably some copy/paste mistake...
git blame says it was me -- sorry! :(
Thanks @athos-ribeiro for spotting it! (#71)
also, add test *.glif files to package_data, so users can run test modules against the installed cu2qu by doing e.g.:
```
python -m cu2qu.test.pens_test
```
