build: Allow release candidate (rc) versions

After this, running `bump2version rc` will increment the `rc` value if
the current version is an rc. While `bump2version` has a good way to
handle a release process that always goes through a release candidate
phase, it doesn't have good support for an optional release candidate
phase.

For example, if the current version is 6.45.0, `bump2version rc` will
change the version to 6.45.0rc1. That would be wrong as 6.45.0rc1 is
older than 6.45.0. Therefore, rc releases should only be done manually
by developers to ensure the new version is correct. See the README for
details.

Note that the trailing space and real tabs in the config file are
intentional as that matches how `ConfigParser` will write back the file
when bumping `current_version`. Without that we'd just get a spurious
whitespace change on the next version bump.

Author: dbnicholson

.bumpversion.cfg

@@ -2,3 +2,7 @@
 current_version = 6.44.0
 commit = True
 tag = True
+parse = (?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)(rc(?P<rc>\d+))?
+serialize = 
+	{major}.{minor}.{patch}rc{rc}
+	{major}.{minor}.{patch}

README.md

@@ -218,6 +218,36 @@ The `bump-version` script can also be run with options such as
 also specify `--current-version` to see the effect without actually
 changing the current version. See `bumpversion --help` for all options.
 
+#### Release candidates
+
+PyPI and `pip` support release candidates with the `rc<N>` suffix. Prior
+to major releases it may be helpful to build and publish release
+candidates so they can be tested prior to general availability. Our
+`bumpversion` configuration has limited support for release candidates,
+so care should be taken to ensure the new version comes out correctly.
+
+To begin a release candidate series, the version must be fully specified:
+
+```bash
+yarn bump-version major "Komodo Dragon" --new-version 7.0.0rc1
+```
+
+To make the next release candidate, run:
+
+``` bash
+yarn bump-version rc
+```
+
+Finally, to end a release candidate series, the version must be fully
+specified again:
+
+``` bash
+yarn bump-version major "Komodo Dragon" --new-version 7.0.0
+```
+
+In all cases, the commit and tag must be pushed to the remote as
+explained above.
+
 ### How to display the build information
 
 To show the build information in the front page, as a tag in top right corner,

scripts/bump_version.py

@@ -33,7 +33,7 @@ def _update_version_name(version_name):
     ap.add_argument(
         "part",
         metavar="PART",
-        choices=("major", "minor"),
+        choices=("major", "minor", "rc"),
         help="Part of the version number to be bumped",
     )
     ap.add_argument(