Pin gevent to latest version 20.4.0

[pyup-bot]

This PR pins gevent to the latest release 20.4.0.

Changelog

20.04.0

====================


Features
--------

- Let CI (Travis and Appveyor) build and upload release wheels for
Windows, macOS and manylinux. As part of this, (a subset of) gevent's
tests can run if the standard library's ``test.support`` module has
been stripped.
See :issue:`1555`.
- Update tested PyPy version from 7.2.0 on Windows to 7.3.1.
See :issue:`1569`.


Bugfixes
--------

- Fix a spurious warning about watchers and resource leaks on libuv on
Windows. Reported by Stéphane Rainville.
See :issue:`1564`.
- Make monkey-patching properly remove ``select.epoll`` and
``select.kqueue``. Reported by Kirill Smelkov.
See :issue:`1570`.
- Make it possible to monkey-patch :mod:`contextvars` before Python 3.7
if a non-standard backport that uses the same name as the standard
library does is installed. Previously this would raise an error.
Reported by Simon Davy.
See :issue:`1572`.
- Fix destroying the libuv default loop and then using the default loop
again.
See :issue:`1580`.
- libuv loops that have watched children can now exit. Previously, the
SIGCHLD watcher kept the loop alive even if there were no longer any
watched children.
See :issue:`1581`.


Deprecations and Removals
-------------------------

- PyPy no longer uses the Python allocation functions for libuv and
libev allocations.
See :issue:`1569`.


Misc
----

- See :issue:`1367`.


----

Links

• PyPI: https://pypi.org/project/gevent
• Changelog: https://pyup.io/changelogs/gevent/
• Homepage: http://www.gevent.org/