* [BREAKING CHANGE] Refactoring: Split event loop in several modules

* `urwid.main_loop` is split into multiple modules which is easier to maintain
* `urwid.compat` is not used anymore and removed
* `TornadoEventLoop`, `GLibEventLoop`, `TwistedEventLoop` and `TrioEventLoop`
  accessible ONLY if required dependencies installed
  (like: Tornado installed -> `TornadoEventLoop` is accessible for import)
* `TornadoEventLoop` use the same idle logic as `AsyncioLoop`:
  tornado.ioloop.IOLoop is asyncio based.
* Trio < 0.15 is not supported. Version 0.15 was released almost 3 years ago.
* Tornado < 5.0 is not supported. Tornado 5.0 was released 5 years ago.

* Remove useless shebang

* `EventLoop` should be real abstract
* add new module docstrings

* Fix docstrings

* remove unneeded import

---------

Co-authored-by: Aleksei Stepanov <alekseis@nvidia.com>

* Initial migration to the python 3.7: Semi-automatic changes

CI related:
Update `tox.ini` and `.travis.yml` to run python3 only tests
Python 3.11 tests is commented-out on travis until #517 is not merged

Manual changes:
* `setup.py`: classifiers, remove python2 compatibility code
* `docs/manual/wcur2.py`: looks like file was never completed, syntax is invalid
* `urwid.compat`: removed `ord2`, `bytes3`, `text_type`, `xrange` and `text_types`

Automatic changes (no manual editing, AST validated equality:
* removed `u` prefix from all strings: not allowed in modern python code
* `bytes()` -> `b''`
* `== None` -> `is None`
* subclassing of `object`
* `super(<Class>`, self>)` ->`super()`
* `from __future__ import ...` python3 compatibility imports
* `set(<Iterable[Hashable]>)` -> `{<Hashable>}`
* partial f-strings conversion
* (`IOError`, `select.error`, `socket.error`) -> `OSError`

* Switch to f-strings (automatic changes)

* Remove `urwid.compat.B`

* Remove `urwid.compat.with_metaclass`

* use native `super()` instead of `self.__super`

* Remove `urwid.compat.chr2`

* Remove `urwid.split_repr.python3_repr`

* Use native `@classmethod` and `@property` where overload is not possible

* Add `from __future__ import annotations`

* automatically sort imports

* Add DeprecationWarning to the deprecated methods

most IDE's will recognize it and annotate during new code usage
call with "warnings as errors" mode will help to refactor other users

* Address comments

* replace homepage address in all files
* remove outdated comments in compat.py
* make wcur2.py correct python code. For example subclass
* replace `self.__super` by `super()` in examples
* fix asyncio_socket_server.py: magic with `asyncio` became wrong

* Remove `widget.update_wrapper`: this was backport of python `functools.update_wrapper`

* display_common.py: fix trivial typo in _colors calculation

* use `sorted` method instead of list construction with later sorting

* Address comments

* `wcur2` include in docs
* warning on `signals.Signals.emit`

---------

Co-authored-by: Aleksei Stepanov <alekseis@nvidia.com>

defindex.html was deprecated a long time ago, see

https://github.com/python/cpython/commit/7970cd483346dfd7723da214fb27399ecc574095

for example.

Co-authored-by: Jochen Sprickerhof <jspricke@debian.org>

"will be have" → "will have"

fix small typo in tutorial