* Fix incorrect type cast in vterm (`apply_mapping` should return `bytes`)

Add `time.sleep(0.1)` to the event loop tests:
  in the worst scenario on windows and slow machine
  function in parallel thread/async can wait up to 80 milliseconds (tested)
Add type annotations to the `vterm` and `test_vterm` to simplify error lookup.
* Fix `DeprecationWarning` in doctests & examples
* Add `pytest` configuration in `pyproject.toml` without migration
* `Signals.emit()` rework: stop `user_args` join with `weak_args`

Partial: #544
Partial: #512
Partial: #406

* drop `sleep`: not enough effective with pytest

* set timer for errors raise to 0: faster raise, faster test done

---------

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

* Deprecate legacy property creation

* Drop long time ago removed methods (never returning methods)
* Move large part of property implementations under `@property`
* Emit `PendingDeprecationWarning`
  for old compatibility code
  for public methods used as core for property and methods for compatibility
* Emit `DeprecationWarning`
  for private methods used in property construction
  using `property()` call

Due to amount of copy-paste like changes, for containers shared part is moved to the existing base classes
Add `__len__` to the list based containers. Related #445
Fix typo in type annotation for `Frame.mouse_event`

* Update urwid/canvas.py

Co-authored-by: Ian Ward <ian@excess.org>

* Update urwid/canvas.py

Co-authored-by: Ian Ward <ian@excess.org>

* Update urwid/tests/test_container.py

Co-authored-by: Ian Ward <ian@excess.org>

* Fix typo in test name

* Frame `header`, `body` and `footer` also has property and methods from pre-property era

Make consistent with other containers

---------

Co-authored-by: Aleksei Stepanov <alekseis@nvidia.com>
Co-authored-by: Ian Ward <ian@excess.org>

* 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>

Identity check (`is`, `is not`) with certain literals (e.g. strings,
numbers) is not guaranteed by the language specification to work and
issue a warning starting with Python 3.8, e.g.:

> examples/browse.py:326: SyntaxWarning: "is" with a literal. Did you mean "=="?

tour.py works with py2 now
Typo in tour.py