- Make versions of the form '1.22.0.dev0' valid for non-releases.
- Put empty match at end of groups instead of at the beginning.
- Require eol in match, do not allow trailing characters.

The new tags look like '1.21.0.dev0+98.gaa0453721f', where '98' is the
number of commits since the 1.21.0 branch was started and 'aa0453721f'.
The chosen form may be specified in the 'setup.cfg' file. This PR adds
two new files 'numpy/_version.py' and 'numpy/version.py'. The latter
is kept because it is part of the public API and is actually used by
some downstream projects, but it is no longer dynamically created.

See https://github.com/python-versioneer/python-versioneer/ for more
information.

As numpy is Python 3 only, these import statements are now unnecessary
and don't alter runtime behavior.

That function is nose specific and has not worked since `__init__` files
were added to the tests directories.

Addresses comments of @pv on gh-6895.

Implements idea suggested in gh-6431.