Use pre instead of ~pre suffix for pre-release versions

Because Setup Tools doesn't support the ~pre suffix anymore:
- https://stackoverflow.com/a/75671981/470844
- pypa/setuptools#3772 (comment)

README.md

@@ -179,13 +179,13 @@ to test your local copy of `fluxgui` without installing anything.
 
 Note changes in `./debian/changelog`.
 
-Use version `<ver>~pre` until ready to release a version. When
-releasing a version remove the `~pre` suffix from the version strings
+Use version `<ver>pre` until ready to release a version. When
+releasing a version remove the `pre` suffix from the version strings
 and commit, copying the changelog changes for the current release into
 the commit message. Then `git tag -a v<ver>`, using the commit msg for
 the tag annotation, and push the version tag with `git push origin
 v<ver>`. Finally, create another commit with the new `<next
-version>~pre` version strings and changelog entry.
+version>pre` version strings and changelog entry.
 
 When releasing the version string needs to be changed in
 `debian/changelog` and `setup.py`, and the release dates needs to be

debian/changelog

@@ -1,5 +1,5 @@
 # -*- text -*-
-fluxgui (2.0.1~pre) wily; urgency=low
+fluxgui (2.0.1pre) wily; urgency=low
 
   * fix buggy appindicator import
   https://github.com/xflux-gui/fluxgui/commit/a21f55fc61f596cfc74e22bef3aaa3ad761f8393

setup.py

@@ -82,7 +82,7 @@ def run(self):
             self.spawn(["glib-compile-schemas", gschema_dir])
 
 setup(name = "f.lux indicator applet",
-    version = "2.0.1~pre",
+    version = "2.0.1pre",
     description = "f.lux indicator applet - better lighting for your computer",
     author = "Kilian Valkhof, Michael and Lorna Herf, Josh Winters",
     author_email = "[email protected]",