Cleaned up the release notes.

By unescaping backslash escapes in a treeprocessor, the text is properly
escaped during serialization. Fixes #1131.

As it is recognized that various third-party extensions may be calling the
old class at `postprocessors.UnescapePostprocessor` the old class remains
in the codebase, but has been deprecated and will be removed in a future
release. The new class `treeprocessors.UnescapeTreeprocessor` should be
used instead.

Lines in 409 and 410 are part of an if-condition-block. However, they are called identically L411 and L412 outside of the if-block anyways. Fixes #1267.

This completely removes all objects which were deprecated in version 3.0 (this change will be included in version 3.4). Given the time that has passed, and the fact that older unmaintained extensions are not likely to support the new minimum Python version, this is little concern about breaking older extensions.

Fixes #1263.

Fixes #1261.

Python dropped support on 2021-12-23. Our policy (#760) is to drop
support on the next point release after Python does.

* Remove py36 tests
* Test multiple recent versions of pypy
* Remove pep562 backport

* Add an extra option `lang_str` to pass the language of the code block
to the specified Pygments formatter.
* Include an example custom Pygments formatter in the documentation
that includes the language of the code in the output using the new option.

Resolves #1255.