Python 37+ initial migration (#522)

* 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* remove outdated comments in compat.py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>

1 files changed, 6 insertions, 4 deletions

diff --git a/docs/tutorial/sig.py b/docs/tutorial/sig.py
index c01c1b2..383553e 100644
--- a/docs/tutorial/sig.py
+++ b/docs/tutorial/sig.py
@@ -1,15 +1,17 @@
+from __future__ import annotations
+
 import urwid
 
 palette = [('I say', 'default,bold', 'default', 'bold'),]
-ask = urwid.Edit(('I say', u"What is your name?\n"))
-reply = urwid.Text(u"")
-button = urwid.Button(u'Exit')
+ask = urwid.Edit(('I say', "What is your name?\n"))
+reply = urwid.Text("")
+button = urwid.Button('Exit')
 div = urwid.Divider()
 pile = urwid.Pile([ask, div, reply, div, button])
 top = urwid.Filler(pile, valign='top')
 
 def on_ask_change(edit, new_edit_text):
-    reply.set_text(('I say', u"Nice to meet you, %s" % new_edit_text))
+    reply.set_text(('I say', f"Nice to meet you, {new_edit_text}"))
 
 def on_exit_clicked(button):
     raise urwid.ExitMainLoop()