Fix typos in the Lib directory as identified by codespell.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>.
(cherry picked from commit 745c9d9dfc1ad6fdfdf1d07420c6273ff67fa5be)

Co-authored-by: Christian Clauss <cclauss@me.com>

Instead of explicitly enumerate test classes for run_unittest()
use the unittest ability to discover tests. This also makes these
tests discoverable and runnable with unittest.

load_tests() can be used for dynamic generating tests and adding
doctests. setUpModule(), tearDownModule() and addModuleCleanup()
can be used for running code before and after all module tests.
(cherry picked from commit 40348acc180580371d25f75f46b27048e35f2435)

(cherry picked from commit 149addd4960d634ce672ab5fc17e0e785a0cdcd0)

Co-authored-by: Zsolt Dollenstein <zsol.zsol@gmail.com>

(cherry picked from commit be42c06bb01206209430f3ac08b72643dc7cad1c)

Co-authored-by: Noah Kantrowitz <noah@coderanger.net>

Add positional only args support to lib2to3 pgen2.

This adds 3.8's PEP-570 support to lib2to3's pgen2.  lib2to3, while
being deprecated is still used by things to parse all versions of Python
code today.  We need it to support parsing modern 3.8 and 3.9 constructs.

Also add tests for complex *expr and **expr's.

Deprecate lib2to3 module in light of PEP 617.

We anticipate removal in the 3.12 timeframe.

There are two copies of the grammar -- the one used by Python itself as
Grammar/Grammar, and the one used by lib2to3 which has necessarily diverged at
Lib/lib2to3/Grammar.txt because it needs to support older syntax an we want it
to be reasonable stable to avoid requiring fixer rewrites.

This brings suport for syntax like `if x:= foo():` to match what the live
Python grammar does.

This should've been added at the time of the walrus operator itself, but lib2to3 being
independent is often overlooked.  So we do consider this a bugfix rather than enhancement.