Partially implement SF feature request 444708.

Add optional arg to string methods strip(), lstrip(), rstrip().
The optional arg specifies characters to delete.

Also for UserString.

Still to do:

- Misc/NEWS
- LaTeX docs (I did the docstrings though)
- Unicode methods, and Unicode support in the string methods.

Original patches were:
python/dist/src/Lib/test/string_tests.py:1.12