remove bumpversion prerelease configuration

I propose to remove the prerelease configuration from bumpversion,
because I think its behavior is just too confusing.

The rational for this is that making the release procedure predictable
is more important than facilitating making pre-releases, which are sort
of the exception in the workflow.

The current configuration makes most common cases confusing:

* bug fix releases require you have to remember to use `--serialize "{major}.{minor}.{patch}"`
* to start a pre-release cycle, you actually use `minor` or `patch`
* to do the actual minor or patch release, you use `prerel`

Also, `prerel` breaks if you run it on a branch with a final release,
because it can't parse the prerelease information.

Therefore, I propose keeping the bumpversion defaults, and do the
prereleases (dev1, dev2, rc1, etc) manually (with `--new-version`),
which makes for a more predictable and intuitive behavior.

* `bumpversion minor` and `bumpversion patch` will work as expected
* pre-releases will be manually handled, but this seems a small overhead
  than remembering the details I mention above.

If you're happy with this, I'll also update [the wiki][1] with new
instructions.

[1]: https://github.com/scrapy/scrapy/wiki/Scrapy-release-procedure

.bumpversion.cfg

@@ -3,27 +3,5 @@ current_version = 1.2.0dev2
 commit = True
 tag = True
 tag_name = {new_version}
-parse = ^
-	(?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)
-	(?:(?P<prerel>[abc]|rc|dev)(?P<prerelversion>\d+))?
-serialize = 
-	{major}.{minor}.{patch}{prerel}{prerelversion}
-	{major}.{minor}.{patch}
 
 [bumpversion:file:scrapy/VERSION]
-
-[bumpversion:part:prerel]
-optional_value = gamma
-values = 
-	dev
-	rc
-	gamma
-
-[bumpversion:part:prerelversion]
-values = 
-	1
-	2
-	3
-	4
-	5
-