Replace use of deprecated Python 'imp' module with 'importlib'

Replace use of deprecated Python 'imp' module with 'importlib'

In the test logs, I've found a number of:

    [...]/tests/mockfedabipkgdiff:42: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
      import imp

I've asked The Internet what to do about that, and this commit is the result.
But beware: I'm not a Python wizard.

	* tests/mockfedabipkgdiff.in: Replace use of deprecated Python
	'imp' module with 'importlib'.

CC: Chenxiong Qi <cqi@redhat.com>
Signed-off-by: Thomas Schwinge <thomas@codesourcery.com>