Porting notes:

- tox handles Python environments automatically. Remove a bit of PYTHONPATH
  manipulation (that was using Python 2 code which always failed!)

- No `clean` target: `git clean -xdf` should fit the bill.

- No `reindent` target: the `reindent.py` script it was using does not
  exist (anymore?).

- No equivalent of tox-test-coverage, which was an artifact of the past,
  using nose.  Instead, the test-coverage target only is ported, which
  uses pytest, and works.

Use a unified script, to reduce code duplication and in preparation
for doing a similar thing with styles and filters.  The new script
also uses a bit more modern Python APIs (e.g., pathlib).

Unlike the previous scripts, it does not replace replace CRLF with LF
because Git should do that itself.

* Add formatter for Pango Markup

* Code style improvement

* Code improvement + test case

* Changed test

* retrigger checks

* Make test independent of color style

pyupgrade is a tool to automatically upgrade syntax for newer versions
of the Python language.

The project has been Python 3 only since
35544e2fc6eed0ce4a27ec7285aac71ff0ddc473, allowing for several cleanups:

- Remove unnecessary "-*- coding: utf-8 -*-" cookie. Python 3 reads all
  source files as utf-8 by default.

- Replace IOError/EnvironmentError with OSError. Python 3 unified these
  exceptions. The old names are aliases only.

- Use the Python 3 shorter super() syntax.

- Remove "utf8" argument form encode/decode. In Python 3, this value is
  the default.

- Remove "r" from open() calls. In Python 3, this value is the default.

- Remove u prefix from Unicode strings. In Python 3, all strings are
  Unicode.

- Replace io.open() with builtin open(). In Python 3, these functions
  are functionally equivalent.

Co-authored-by: Matthäus G. Chajdas <Anteru@users.noreply.github.com>

* Remove Python 2 compatibility

* remove 2/3 shims in pygments.util
* update setup.py metadata

* Remove unneeded object inheritance.

* Remove unneeded future imports.