Add --final option to bumpversion command.

[mauritsvanrees]

This removes alpha / beta / rc markers from the version.

Use case: Plone 6.0 has a first release candidate and the version pins contain a lot of alpha, beta, and rc releases. Before the final release, I want to make final releases of the dependencies. I know I won't remember to do this manually for all packages, so I automate it. :-)

Output from a sample run in the Plone package, redacted for brevity/clarity:

$ bumpversion --final
Checking version bump for final release.
Last tag: 6.0.0rc1
Current version: 6.0.0rc2.dev0
Enter version [6.0.0.dev0]:
INFO: Set setup.cfg's version to '6.0.0.dev0'
INFO: Changed version from 6.0.0rc2.dev0 to 6.0.0.dev0
INFO: History file CHANGES.rst updated.
INFO: The 'git diff':

diff --git a/CHANGES.rst b/CHANGES.rst

-6.0.0rc2 (unreleased)
----------------------
+6.0.0 (unreleased)
+------------------

diff --git a/setup.cfg b/setup.cfg

 [metadata]
-version = 6.0.0rc2.dev0
+version = 6.0.0.dev0