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>

* improve fenced code recognition for markdown lexer

* improve inline code detection

* improve detection of some Markdown keywords

* remove Markdown recognition of code indented by 4 spaces as reliable detection is not possible with regex

* Add support for Setext-style headings in Markdown

* Improve inline code detection in Markdown

* Add support for indented code blocks in Markdown

* Improve italics & bold detection in Markdown

* Simplify italics & bold regexes in Markdown

* Add warning about possible unrecognized internal tags in Markdown

* Improve striktethrough detection in Markdown

* Small bugfix in Markdown

* Small bugfix in Markdown

* Small refactoring in Markdown

Fixes the bug reported in #1389, where the markdown lexer will not issue a
token for the closing code fence.

Issue: #1389