Merging part of Feb 27, 2021 upstream commit

This update is largely about the migration from Python 2 to Python 3.
Documentation links were updated. Sections of code that were marked as
"Added in Python 3" are merged into the main section, as they now
represent the status quo.

Changes can chiefly be described as:
+ the list of builtins and exceptions is updated
+ backwards-compatible highlighting of exec is abandoned (it is no
  longer a command)
  + exec was always a terrible idea anyway, so the cost is negligible
+ backwards-compatible highlighting of print is abandoned (it is not
  longer a command)
  + the print function has been available since Python 2.6

Triple-quoted strings are comments not tests

The paradigm of using triple-quoted strings as unit tests is outdated
and strange. All triple-quoted strings are comments only.

This simplifies a great deal of pattern matching, leading to a more
reliable experience.

Initial commit following rebuild