Brand as "3.8", rework %python_provide
    
    - Hardcode the default Python 3 version in the SRPM macros
    - Provide python38-foo for python3-foo and the other way around (future RHEL compatibility)
    
    Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1812087
    
        $ rpm --eval '%python_provide python38-setuptools'
        Provides: python-setuptools = %{version}-%{release}
        Provides: python3-setuptools = %{version}-%{release}
        Obsoletes: python-setuptools < %{version}-%{release}
    
        $ rpm --eval '%python_provide python3-setuptools'
        Provides: python-setuptools = %{version}-%{release}
        Provides: python38-setuptools = %{version}-%{release}
        Obsoletes: python-setuptools < %{version}-%{release}
    
        $ rpm --eval '%python_provide python39-setuptools'
    
        $ rpm --define 'python3_pkgversion 39' --eval '%python_provide python%{python3_pkgversion}-setuptools'
    
    To make the implementation of %python_provide easier,
    any names starting with "python" or "pypy" are recognized as valid arguments.
    
    Previously, this was an ERROR:
    
        $ rpm --eval '%python_provide pythonista'
        %python_provide: ERROR: pythonista not recognized.
    
    Now it is a no-op. The behavior was never documented and the change is
    backwards compatible for working spec files.