Switch to versioneer for internal python versions

[djhoese]

SatPy and other pytroll packages have switched to using the versioneer tool for identifying the version of a python package. This tool lets you "tag" a release in git which will automatically make the python version python -c 'import polar2grid; print(polar2grid.__version__)' match the name of the last git tag. This goes further where if the installed version isn't tagged (v3.0.0) then it gets the git commit hash added to the end of it (v3.0.0+abcdefg). This is extremely helpful in identifying if one installed version is the same as a released package.

[djhoese]

See pytroll/satpy#678 for details but it may be better to use setuptools-scm instead.