[Python-ideas] Add `rc` to distutils.version.StrictVersion

[Pål Grønås Drange]

StrictVersion from distutils accepts version tags like
1.14.0
1.14.0a1
1.14.0b2

but not
1.14.0rc1 (nor 1.14.0c1).

My suggestion:  Add `rc` in the regexp and make it a `prerelease` (the
latter comes for free by the current implementation).

Most package maintainers have adopted the `rc` abbreviation for release
candidate versioning, e.g.
- numpy 1.14.0rc1
- scipy 1.1.0rc1
- plotly 3.0.0rc1
- pandas 0.23.0rc1
- matplotlib 2.2.0rc1
- dask 0.13.0rc1
- django 1.9rc1.
All of these are available on PyPI.

A natural way of sorting version numbers from pip is by simply using
    sorted(versions, key=distutils.version.StrictVersion),
however, due to StrictVersion only accepting `a` and `b` as
abbreviations, this does not work for the aforemention packages.

The very obvious cons are:
- touching 20 years old code [1]
- StrictVersion is preserved "for anal retentives and software
  idealists", and I don't know if they agree.

There might be more cons I fail to think of at this moment.

[1]
https://github.com/python/cpython/blob/master/Lib/distutils/version.py#L130

Pål Grønås Drange
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