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.
diff --git a/Lib/test/string_tests.py b/Lib/test/string_tests.py
index 4b2f0e3..5c8dd93 100644
--- a/Lib/test/string_tests.py
+++ b/Lib/test/string_tests.py
@@ -163,6 +163,18 @@
     test('rstrip', '   hello   ', '   hello')
     test('strip', 'hello', 'hello')
 
+    # strip/lstrip/rstrip with None arg
+    test('strip', '   hello   ', 'hello', None)
+    test('lstrip', '   hello   ', 'hello   ', None)
+    test('rstrip', '   hello   ', '   hello', None)
+    test('strip', 'hello', 'hello', None)
+
+    # strip/lstrip/rstrip with real arg
+    test('strip', 'xyzzyhelloxyzzy', 'hello', 'xyz')
+    test('lstrip', 'xyzzyhelloxyzzy', 'helloxyzzy', 'xyz')
+    test('rstrip', 'xyzzyhelloxyzzy', 'xyzzyhello', 'xyz')
+    test('strip', 'hello', 'hello', 'xyz')
+
     test('swapcase', 'HeLLo cOmpUteRs', 'hEllO CoMPuTErS')
     test('translate', 'xyzabcdef', 'xyzxyz', transtable, 'def')