Documentation updates for issues 8302 and 8351 (truncating excessive diffs in unittest failure messages and reporting SkipTest exceptions in setUpClass and setUpModule as skips rather than errors).

Tests for issue 8302, skipped test in a setUpClass or a setUpModule are reported as skips rather than errors.

Issue 8302. SkipTest exception is setUpClass or setUpModule is now reported as a skip rather than an error.

1) #8271: when a byte sequence is invalid, only the start byte and all the
   valid continuation bytes are now replaced by U+FFFD, instead of replacing
   the number of bytes specified by the start byte.
   See http://www.unicode.org/versions/Unicode5.2.0/ch03.pdf (pages 94-95);
2) 5- and 6-bytes-long UTF-8 sequences are now considered invalid (no changes
   in behavior);
3) Add code and tests to reject surrogates (U+D800-U+DFFF) as defined in
   RFC 3629, but leave it commented out since it's not backward compatible;
4) Change the error messages "unexpected code byte" to "invalid start byte"
   and "invalid data" to "invalid continuation byte";
5) Add an extensive set of tests in test_unicode;
6) Fix test_codeccallbacks because it was failing after this change.

unittest.TestCase assertion methods inform you when they have omitted an over long diff on failure. Issue 8351.

unittest.TestCase.assertDictEqual and assertMultilineEqual provide better default failure messages in the event of long diffs.

This is just a quick fix:  if the warning is turned into an exception, the
exception simply gets ignored.

Removed the new max_diff argument to assertSequenceEqual. All unittest.TestCase assert methods that use difflib to produce failure messages now truncate overly long messages. New class attribute unittest.TestCase.maxDiff to configure this if necessary. Issue 8351.

unittest TestLoader test discovery filename matching done in a method. This makes it easier to override the matching strategy in subclasses. No behaviour change in actual implementation.

Previously, unexpected results occurred when email was passed, for example,
'utf8' as a charset name, since email would accept it but would *not* use
the 'utf-8' codec for it, even though Python itself recognises that as
an alias for utf-8.  Now Charset checks with codecs for aliases as well
as its own internal table.  Issue 8898 has been opened to change this
further in py3k so that all aliasing is routed through the codecs module.