[2.7] bpo-30759: Fix test_multibytecodec_support

[vstinner]

TestBase_Mapping doesn't inherit from unittest.TestCase anymore, but
object, since it's not a real test case but only a base base. All
child classes already inherit from unittest.TestCase.

[vstinner]

Example: test_codecmaps_tw still runs the same 6 tests with this change.

haypo@selma$ ./python -m test -v test_codecmaps_tw
...
test_errorhandle (test.test_codecmaps_tw.TestBIG5Map) ... ok
test_mapping_file (test.test_codecmaps_tw.TestBIG5Map) ... ok
test_mapping_supplemental (test.test_codecmaps_tw.TestBIG5Map) ... ok
test_errorhandle (test.test_codecmaps_tw.TestCP950Map) ... ok
test_mapping_file (test.test_codecmaps_tw.TestCP950Map) ... ok
test_mapping_supplemental (test.test_codecmaps_tw.TestCP950Map) ... ok
...
Ran 6 tests in 2.363s

[serhiy-storchaka]

Yes, this works.

But maybe make the tests more similar to 3.x and just rename test_multibytecodec_support.py to multibytecodec_support.py.

[serhiy-storchaka]

See 75d9aca.

[serhiy-storchaka]

Backported in #2548.

[vstinner]

Abandonned in favor of @serhiy-storchaka PR #2548.