tools: Add gen-cpydiff.py to generate docs differences.

This patch introduces the a small framework to track differences between
uPy and CPython.  The framework consists of:

- A set of "tests" which test for an individual feature that differs between
  uPy and CPy.  Each test is like a normal uPy test in the test suite, but
  has a special comment at the start with some meta-data: a category (eg
  syntax, core language), a human-readable description of the difference, a
  cause, and a workaround.  Following the meta-data there is a short code
  snippet which demonstrates the difference.  See tests/cpydiff directory
  for the initial set of tests.

- A program (this patch) which runs all the tests (on uPy and CPy) and
  generates nicely-formated .rst documenting the differences.

- Integration into the docs build so that everything is automatic, and the
  differences appear in a way that is easy for users to read/reference (see
  latter commits).

The idea with using this new framework is:

- When a new difference is found it's easy to write a short test for it,
  along with a description, and add it to the existing ones.  It's also easy
  for contributors to submit tests for differences they find.

- When something is no longer different the tool will give an error and
  difference can be removed (or promoted to a proper feature test).

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