bpo-39939: Add str.removeprefix and str.removesuffix (GH-18939)

Added str.removeprefix and str.removesuffix methods and corresponding
bytes, bytearray, and collections.UserString methods to remove affixes
from a string if present. See PEP 616 for a full description.
diff --git a/Lib/test/test_doctest.py b/Lib/test/test_doctest.py
index 7142627..16196fe 100644
--- a/Lib/test/test_doctest.py
+++ b/Lib/test/test_doctest.py
@@ -665,7 +665,7 @@
 
     >>> import builtins
     >>> tests = doctest.DocTestFinder().find(builtins)
-    >>> 810 < len(tests) < 830 # approximate number of objects with docstrings
+    >>> 816 < len(tests) < 836 # approximate number of objects with docstrings
     True
     >>> real_tests = [t for t in tests if len(t.examples) > 0]
     >>> len(real_tests) # objects that actually have doctests