Issue #28691: Fix warn_invalid_escape_sequence(): handle correctly
DeprecationWarning raised as an exception. First clear the current exception to
replace the DeprecationWarning exception with a SyntaxError exception.

Unit test written by Serhiy Storchaka.

And most of the tools.

Patch by Emanual Barry, reviewed by me, Serhiy Storchaka, and
Martin Panter.

I have compared output between pre- and post-patch runs of these tests
to make sure there's nothing missing and nothing broken, on both
Windows and Linux.  The only differences I found were actually tests
that were previously *not* run.

and a full traceback including line number.

Fix "sys.path modified" warning in test_strlit, by not replacing sys.path itself, only its contents.

'latin-1' and 'utf-8'.

These are optimized in the Python Unicode implementation
to result in more direct processing, bypassing the codec
registry.

Also see issue11303.

raw string literals.  I added a whole bunch of tests but am still not sure
I am testing all paths through the code.  I really think the code could be
simplified quite a bit.