Remove own usage of incremental.

[adiroiban]

Description

Fixes #308

To simplify the number of dependencies towncrier can use a string as its version.

Also, when finding the version of a project, just use str() on the project version instancce.

project using incremental are still supported, as str(incremental.Version) does not return what we want... and is ok to keep incrementla for backwards compatibility.

Todo

See if version is a tuple and join them with a dot.
For anything else, just call str(version_object) and strip it.

Checklist

• Make sure changes are covered by existing or new tests.
• For at least one Python version, make sure local test run is green.
• Create a file in src/towncrier/newsfragments/. Describe your
  change and include important information. Your change will be included in the public release notes.
• Make sure all GitHub Actions checks are green (they are automatically checking all of the above).

[hynek]

Do you still want to do this Adi? Or wanna start fresh? (I support the change, but this branch is very stale)

[adiroiban]

I still would like to have a simple string as the version for towncrier.
Right now, I don't have the motivation and time to continue the work.

[adiroiban]

This can be closed as there is #491