Set %__python3 value according to %python3_pkgversion
    
    I.e. when %python3_pkgversion is 3.12, %__python3 is /usr/bin/python3.12
    
    We assume that when packagers pacakge for Python 3.X, they want to change both
    %python3_pkgversion and %__python3 value.
    
    Hence instead of copy-pasting this:
    
        %global python3_pkgversion 3.X
        %global __python3 /usr/bin/python3.X
    
    They just need to do:
    
        %global python3_pkgversion 3.X
    
    Packagers who want to change the value of %__python3 without touching
    %python3_pkgversion can still do it:
    
        %global __python3 /usr/bin/pypy3
    
    Related to https://bugzilla.redhat.com/1821489