Nox

- Hide the python interpreter on sessions with only one. (148)
- Warn when programs not in the virtualenv are used, allow erroring and silencing the warning. (147)
- Add --warn-on-external-run flag and the "external" keyword arg to session.run. (147)
- Add nox.options which allows specifying command-line configuration in the Noxfile (145)
- Add python_requires (>= 3.5) to setup.py.

Breaking changes:

- Skip sessions with missing interpreters. Previously, missing interpreters would cause a failed session. Now they just cause a warning. The previous behavior can be used via `--error-on-missing-interpreters`. (140)

New features:

- Add session.python property.

Other changes:

- Fix some warnings about escape sequences.
- Group command line args by usage for readability.
- Blacklist more Tox env vars in nox-to-tox.
- Documentation fixes, spelling, etc. (134)
- Mention stickers in the contributors guide.
- Mention Invoke as a Nox alternative.

- Check for ``NOXSESSION`` environment variable (121)
- Fix typo in OpenCensus Python (132)
- Add new documentation art created by Andrea Caprotti
- Add Python 3.7 to Travis CI (129)

Bugfixes:

* Adding `Session.__slots__`. (128)
* Don't modify `Virtualenv.interpreter` in `_resolved_interpreter`. (127)
* Fix tox-to-nox template.
* Add the ability to add descriptions to sessions. (117)
* Using more specific regex in `_resolved_interpreter()`. (119)

New features:

* Adding support for 32-bit binaries on Windows. (100)

Internal/testing changes:

* Storing `platform.system()` as global in `nox.virtualenv`. (124)
* Fix deploy script for Travis.
* Run docs on travis. (122)
* Documentation style updates.

**Heads up!** This is a very big release for Nox. Please read these release notes thoroughly and reach out to us on GitHub if you run into issues.

Breaking changes and other important notes:

* Nox is now published as "nox" on PyPI. This means that Nox is installed via `pip install nox` instead of `nox-automation`. Since the new release makes so many breaking changes, we won't be updating the old `nox-automation` package.
* Nox's configuration file is now called `noxfile.py` instead of `nox.py`.
* Nox no longer supports Python 2.7. You can still create and run Python 2.7 sessions, but Nox itself must be installed using Python 3.5+.
* Nox's behavior has been changed from *declarative* to *imperative*. Session actions now run immediately. Existing code to setup session virtualenv, such as `session.interpreter` **will break**! Please consult the documentation on how to use `nox.session(python=[...])` to configure virtualenvs for sessions.
* Nox now uses calver for releases.
* Support for the legacy naming convention (for example, `session_tests`) has been removed.

Other changes:

* Update colorlog dependency range.
* Update installation command in contributing. (101)
* Remove Python 2.x object inheritance. (109)
* Fix python syntax error on docs. (111)
* Show additional links on PyPI. (108)
* Add contributors file. (113)
* Run sphinx with -W option. (115)
* Using `os.pathsep` instead of a hardcoded `':'`. (105)
* Use a configuration file for readthedocs. (107)
* Add 'py' alias for `nox.session(python=...)`.
* Fix processing of numeric Python versions that specify a patch version.
* Use Black to format code.
* Support invoking Nox using `python -m nox`.
* Produce better error message when sessions can't be found.
* Fix missing links in README.rst.
* Remove usage of future imports.
* Remove usage of six.
* Make session.install a simple alias for session.run.
* Refactor nox.command.Command as nox.command.run, reducing complexity.
* Add list of projects that use Nox.
* Use witchhazel pygments theme.

**Note**:: v0.19.1 was the last version released as "nox-automation" on PyPI. Subsequent releases are published as "nox".

* Updates copyright information and contact addresses. No code changes.