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.

macros.python-srpm

@@ -25,6 +25,11 @@
     end\
 }
 
+# This specifies what packages are equal to the python3-... packages, in python_provide macro
+# E.g. in Fedora 32, python38-foo will provide python3-foo and vice versa
+%__default_python3_version 3.8
+%__default_python3_pkgversion %(echo %__default_python3_version | sed 's/\\.//')
+
 # python3_pkgversion specifies the version of Python 3 in the distro.  It can be
 # a specific version (e.g. 34 in Fedora EPEL7)
 %python3_pkgversion 3