Update gunicorn to 20.1.0

[pyup-bot]

This PR updates gunicorn from 20.0.4 to 20.1.0.

Changelog

20.1.0

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

- document WEB_CONCURRENCY is set by, at least, Heroku
- capture peername from accept: Avoid calls to getpeername by capturing the peer name returned by
accept
- log a warning when a worker was terminated due to a signal
- fix tornado usage with latest versions of Django 
- add support for python -m gunicorn
- fix systemd socket activation example
- allows to set wsgi application in configg file using `wsgi_app`
- document `--timeout = 0`
- always close a connection when the number of requests exceeds the max requests
- Disable keepalive during graceful shutdown
- kill tasks in the gthread workers during upgrade
- fix latency in gevent worker when accepting new requests
- fix file watcher: handle errors when new worker reboot and ensure the list of files is kept
- document the default name and path of the configuration file
- document how variable impact configuration
- document the `$PORT` environment variable
- added milliseconds option to request_time in access_log
- added PIP requirements to be used for example
- remove version from the Server header
- fix sendfile: use `socket.sendfile` instead of `os.sendfile`
- reloader: use  absolute path to prevent empty to prevent0 `InotifyError` when a file 
is added to the working directory
- Add --print-config option to print the resolved settings at startup.
- remove the `--log-dict-config` CLI flag because it never had a working format
(the `logconfig_dict` setting in configuration files continues to work)


** Breaking changes **

- minimum version is Python 3.5
- remove version from the Server header 

** Documentation **



** Others **

- miscellaneous changes in the code base to be a better citizen with Python 3
- remove dead code
- fix documentation generation

Links

• PyPI: https://pypi.org/project/gunicorn
• Changelog: https://pyup.io/changelogs/gunicorn/
• Homepage: https://gunicorn.org