Do not use pkg_resources to determine version at runtime

[cgohlke]

Significantly reduces import time

On this Windows workstation, the time for import triangle decreases from ~3.3 s to ~<0.2 s.

See also vispy/vispy#1779, python-lz4/python-lz4#154, and pypa/setuptools#926

[drufat]

I did not realize pkg_resources was so slow.
It would still be nice to follow the Don't Repeat Yourself principle, and have the version number only in one place. Do you think you can move the pkg_resources code and the __version__ variable to a submodule triangle/version.py so that it does not get imported by default? That way I will not have to update the version number in two separate place every time it changes?

[drufat]

I will merge this pull request, and then use the first technique described here:
https://packaging.python.org/guides/single-sourcing-package-version/
to single source the version number in setup.py

[drufat]

I have released a new version. This issue should be fixed now.