Switch version scheme to release-branch-semver.

While setuptools-scm does not yet support our branch naming scheme for
release branches, it will soon [1]. And we won't have to worry about
that until we actually branch for 3.5.0, since the v3.4.x branch doesn't
use it.

[1] pypa/setuptools_scm#573

setup.py

@@ -336,7 +336,7 @@ def make_release_tree(self, base_dir, files):
         else []
     ),
     use_scm_version={
-        "version_scheme": "post-release",
+        "version_scheme": "release-branch-semver",
         "local_scheme": "node-and-date",
         "write_to": "lib/matplotlib/_version.py",
         "parentdir_prefix_version": "matplotlib-",