WIP: Remove python3-mock dependency from fedpkg
    
    This commit introduces use of unittest.mock on Python3.
    
    Jira: RHELCMP-8616
    And https://fedoraproject.org/wiki/Changes/DeprecatePythonMock#How_to_migrate_to_unittest.mock
    Relates: https://bugzilla.redhat.com/show_bug.cgi?id=2061765
    
    Signed-off-by: Dominik Rumian <drumian@redhat.com>