Set %__python3 to /usr/bin/python3.6 by default
    
    When %%python3_pkgveresion is set to 3 (i.e. the default), we override
    %%__python3 to /usr/bin/python3.6 to ensure that programs are always run
    with python3.6, even if /usr/bin/python3 is a symlink to a different
    interpreter.
    
    After:
    
    ```
    $ rpm -E %python3
    /usr/bin/python3.6
    $ rpm -E %__python3
    /usr/bin/python3.6
    $ rpm -D 'python3_pkgversion 3.6' -E %__python3
    /usr/bin/python3.6
    $ rpm -D 'python3_pkgversion 3.8' -E %__python3
    /usr/bin/python3.8
    $ rpm -D 'python3_pkgversion 3.9' -E %__python3
    /usr/bin/python3.9
    $ rpm -D 'python3_pkgversion 3.11' -E %__python3
    /usr/bin/python3.11
    $ rpm -D 'python3_pkgversion 3.11' -E %python3
    /usr/bin/python3.11
    $ rpm -D 'python3_pkgversion 3.6' -E %python3
    /usr/bin/python3.6
    $ rpm -E %python3
    /usr/bin/python3.6
    ```
    
    Before:
    ```
    $ rpm -E %python3
    /usr/bin/python3
    $ rpm -D 'python3_pkgversion 3.8' -E %python3
    /usr/bin/python3.8
    [...]
    ```
    
    Relates: https://lists.fedoraproject.org/archives/list/epel-devel@lists.fedoraproject.org/thread/RE3PG72B5AX7NTACPDSBGOWCMN7I3OQJ/