* Add GAP console session mode

This is also appropriate for GAP .tst files.

Add `analyse_text` methods for `ScilabLexer` and `GAPConsoleLexer` to
distinguish Scilab and GAP .tst files

* Use explicit name for 'keepends' argument to splitlines

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.

* Diff lexer - match whitespace

* Make lexer - whitespace token set

* Actionscript lexer - whitespace token

* Bare lexer - whitespace token

* Business(cobol) lexers - whitespace token

As pointed out by Issue #1860 the keyword unexport was not correctly
matched. Only the part `export' had been matched.

This commit adds an optional group to the regex of the `export`
keyword to match `unexport` as well.

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>