Set Version: %{__default_python3_version} to remove one place to bump
Since Fedora 33, the package version always == %{__default_python3_version}.
When we update Python to 3.X+1, we update the version and the macro.
When the macro is not updated confusing things happen.
See for example https://bugzilla.redhat.com/show_bug.cgi?id=1931421#c4
We could assert the versions match in %check instead,
but this way a temporary pull request for a new Python version, such as
https://src.fedoraproject.org/rpms/python-rpm-macros/pull-request/50https://src.fedoraproject.org/rpms/python-rpm-macros/pull-request/84
can be kept rebased via the git forge UI.
Note that the value of %{__default_python3_version} is loaded from sources in
dist git, otherwise it would be defined by the previous build of this package.
As a result, the spec file is no longer parsable on it's own, but IMHO that's OK.