- Should have just combined those earlier minor linting fixes...

- Also ensure that optimised mode is added to the CI workflow.

- Made use of the more fine-tuned specific flake8 per-file-ignores
  configuration option to limit what formatting issues should be ignored
  for the affected files.
- Also removed all remaining overspecified ignores and corrected the
  missing raw strings prefix for strings that have escape sequences that
  are not currently defined.

- Include additional notable changes.

- Also include a reference to the previous fix.

- Mention something about performance.
- Also noting the changes.

- Also note that this library now has higher level support of sourcemap
  generation as request by rspivak/slimit#66.

This required very substantial changes, some of which were cleanups that
were done on one stab included with this one, namely:

- Refactored the sourcemap.write function, the return value matches what
  documentation for the class said (mapping before names).
- Reordered the arguments for that, too - normalize is now the first
  keyword argument (should really be all keywords, but due to support
  for Python 2, this cannot be specified).

Now that write function and the walk function that feeds into it can be
both corrected in one go:

- Introduce a new ruletype for the final stream output - StreamFragment.
  This is a 5-tuple that includes the source, which will now be derived
  using the sourcepath attributes of input Nodes.
- However, in the interest of flexibility and not clobber the original
  logic completely, the unparser walk function now support a couple
  additional arguments, namely the conversion function which defaults to
  converting the TextChunk (formerly SourceChunk) type into the desired
  StreamFragment type so the source can be tracked.
- The other is a decorator function for the inner walk function which
  defaults to an implementation that will track the stack of Nodes that
  have sourcepath assigned, so that the source lookup can be done by
  checking the most recently pushed value without having to backtracked
  to its parent node for that value.  Not that parent references are
  available, however.
- This also allow the use of an identity function for both these
  arguments to return the walk function to produce the chunks without
  the source handling.  Alternatively provide a way to simply produce
  the text output only.  This is tested by the test_es5_walker module
  so that the SimpleChunk would not be turned into the StreamFragment
  which relies on attributes that it doesn't provide.
- To minimize disruption of existing tests, the test that checks the
  raw output of the Unparser, which is produced through the walk
  function, mostly will turn that back into a 4-tuple for checking
  against the existing values.
- Finally, for cases where nodes do not provide sourcepath, and given
  that the sourcemap requires a path (or url?) for the source, the
  about:invalid will be used (RFC6694, and the iana about-uri-tokens
  registry) to satisfy that value.

- Have the Dispatcher object be able to resolve Deferred types for
  specific handlers, if required.
- This also require a change on how BaseUnparser class construction is
  done, as the callable that set up the handlers will need to be
  generalized.

- So the breaking changes can be done to make life easier.

- Also note that the following issues were addressed, where applicable
  to the lexer or parser.

  - rspivak/slimit#52
  - rspivak/slimit#54
  - rspivak/slimit#57
  - rspivak/slimit#59
  - rspivak/slimit#62
  - rspivak/slimit#65
  - rspivak/slimit#70
  - rspivak/slimit#73
  - rspivak/slimit#79
  - rspivak/slimit#81
  - rspivak/slimit#82
  - rspivak/slimit#90

- Will get the release out when I get some sleep.