Sets up desired warning filters.

If the warning filters are not specified on the command line or via the system variable make sure that DeprecationWarning and ImportWarning raised by this this package always get printed.

The warning settings used by pytest can be found in pytest.ini, where in addition to these two warnings PendingDeprecationWarning is enabled as well.

This functionality is tested by test_warnings_emission1() and test_warnings_emission2() functions in fatf.tests.test_warning_filters module.