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/50
https://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.

python-rpm-macros.spec

@@ -1,11 +1,6 @@
 Name:           python-rpm-macros
-Version:        3.9
-Release:        35%{?dist}
 Summary:        The common Python RPM macros
 
-# macros and lua: MIT, compileall2.py: PSFv2
-License:        MIT and Python
-
 # Macros:
 Source101:      macros.python
 Source102:      macros.python-srpm
@@ -19,6 +14,16 @@ Source201:      python.lua
 %global compileall2_version 0.7.1
 Source301:      https://github.com/fedora-python/compileall2/raw/v%{compileall2_version}/compileall2.py
 
+# macros and lua: MIT, compileall2.py: PSFv2
+License:        MIT and Python
+
+# The package version MUST be always the same as %%{__default_python3_version}.
+# To have only one source of truth, we load the macro and use it.
+# The macro is defined in python-srpm-macros.
+                %{load:%{SOURCE102}}
+Version:        %{__default_python3_version}
+Release:        35%{?dist}
+
 BuildArch:      noarch
 
 # For %%__default_python3_pkgversion used in %%python_provide