This triggers a new case in the HtmlFormatter, which emits an empty span
at the end of the line for a new line, as those are removed by the
split-by-parts code. This requires separate post-processing.

Doesn't fix all whitespace issues with Python either, but we're done to
360 failing examples with that, from previously >400.

Disable highlighting of unicode escape codes in python bytes literals,
as described in
https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals.
So escape codes of the form "\N{name}", "\uxxxx" and "\Uxxxxxxxx" will
no longer be highlighted within bytes literals.  Add tests for escape
code highlighting in string and bytes literals.

EncodingWarning was added in 3.10: https://docs.python.org/3/library/exceptions.html#EncodingWarning

This commit adds a new url field to a lexer, which can be used to link
to the language website, instead of relying on having the link in either
languages.rst or the docstring of the lexer. Additionally, it changes the
languages.rst file to auto-generate the list of lexers from the actual
source code, using the provided URL.