Add docstrings and tests for codecontext.py.

After this is merged, we should immediately add 'async' to the list of block openers. With complete tests, we can more confidently cleanup the code a bit, as well as change behavior. The fourth field of info tuples, block keyword, is never used as part of info tuples. (It is used before being added in get_context.)

Because BLOCKOPENERS is sufficiently distant from the changes for this issue, I will add 'async' now, without tests, in #33564. Some of my comments for this issue are ideas for future changes in codecontext.py.

 New changeset 654038d896d78a8373b60184f335acd516215acd by Terry Jan Reedy (Cheryl Sabella) in branch 'master': bpo-32831: IDLE: Add docstrings and tests for codecontext (GH-5638) https://github.com/python/cpython/commit/654038d896d78a8373b60184f335acd516215acd 

 New changeset 83aedc4d9ae36fb7bc05fd8b2caec55697b3efd2 by Miss Islington (bot) in branch '3.6': bpo-32831: IDLE: Add docstrings and tests for codecontext (GH-5638) https://github.com/python/cpython/commit/83aedc4d9ae36fb7bc05fd8b2caec55697b3efd2 

 New changeset 0efa1353b756bd61d4e852ff4ef146735bef5522 by Miss Islington (bot) in branch '3.7': bpo-32831: IDLE: Add docstrings and tests for codecontext (GH-5638) https://github.com/python/cpython/commit/0efa1353b756bd61d4e852ff4ef146735bef5522 

Thanks for the good set of tests.

If you are following up with a new patch that includes code changes, make a new issue. In the opening message, please briefly list the mini-issues fixed so I can refer changes to purposes. Perhaps something like * getspacesfirstword - function and param1 names ... List the new issue as a dependency of new 'improve code context' issue 33610 that addresses point 2.