Update tox to 3.21.1

[pyup-bot]

This PR updates tox from 3.19.0 to 3.21.1.

Changelog

3.21.1

--------------------

Bugfixes
^^^^^^^^

- Fix regression that broke using install_command in config replacements - by :user:`jayvdb`
`1777 <https://github.com/tox-dev/tox/issues/1777>`_
- Fix regression parsing posargs default containing colon. - by :user:`jayvdb`
`1785 <https://github.com/tox-dev/tox/issues/1785>`_


Features
^^^^^^^^

- Prevent .tox in envlist - by :user:`jayvdb`
`1684 <https://github.com/tox-dev/tox/issues/1684>`_


Miscellaneous
^^^^^^^^^^^^^

- Enable building tox with ``setuptools_scm`` 4 and 5 by :user:hroncok
`1799 <https://github.com/tox-dev/tox/issues/1799>`_

3.21.0

--------------------

Bugfixes
^^^^^^^^

- Fix the false ``congratulations`` message that appears when a ``KeyboardInterrupt`` occurs during package installation. - by :user:`gnikonorov`
`1453 <https://github.com/tox-dev/tox/issues/1453>`_
- Fix ``platform`` support for ``install_command``. - by :user:`jayvdb`
`1464 <https://github.com/tox-dev/tox/issues/1464>`_
- Fixed regression in v3.20.0 that caused escaped curly braces in setenv
to break usage of the variable elsewhere in tox.ini. - by :user:`jayvdb`
`1690 <https://github.com/tox-dev/tox/issues/1690>`_
- Prevent ``{}`` and require ``{:`` is only followed by ``}``. - by :user:`jayvdb`
`1711 <https://github.com/tox-dev/tox/issues/1711>`_
- Raise ``MissingSubstitution`` on access of broken ini setting. - by :user:`jayvdb`
`1716 <https://github.com/tox-dev/tox/issues/1716>`_


Features
^^^^^^^^

- Allow \{ and \} in default of {env:key:default}. - by :user:`jayvdb`
`1502 <https://github.com/tox-dev/tox/issues/1502>`_
- Allow {posargs} in setenv. - by :user:`jayvdb`
`1695 <https://github.com/tox-dev/tox/issues/1695>`_
- Allow {/} to refer to os.sep. - by :user:`jayvdb`
`1700 <https://github.com/tox-dev/tox/issues/1700>`_
- Make parsing [testenv] sections in setup.cfg official. - by :user:`mauvilsa`
`1727 <https://github.com/tox-dev/tox/issues/1727>`_
- Relax importlib requirement to allow 3.0.0 or any newer version - by
:user:`pkolbus`
`1763 <https://github.com/tox-dev/tox/issues/1763>`_


Documentation
^^^^^^^^^^^^^

- Document more info about using ``platform`` setting. - by :user:`prakhargurunani`
`1144 <https://github.com/tox-dev/tox/issues/1144>`_
- Replace ``indexserver`` in documentation with environment variables - by :user:`ziima`.
`1357 <https://github.com/tox-dev/tox/issues/1357>`_
- Document that the ``passenv`` environment setting is case insensitive. - by :user:`gnikonorov`
`1534 <https://github.com/tox-dev/tox/issues/1534>`_

3.20.1

--------------------

Bugfixes
^^^^^^^^

- Relax importlib requirement to allow version<3 - by :user:`usamasadiq`
`1682 <https://github.com/tox-dev/tox/issues/1682>`_

3.20.0

--------------------

Bugfixes
^^^^^^^^

- Allow hyphens and empty factors in generative section name. - by :user:`tyagdit`
`1636 <https://github.com/tox-dev/tox/issues/1636>`_
- Support for PEP517 in-tree build backend-path key in ``get-build-requires``. - by :user:`nizox`
`1654 <https://github.com/tox-dev/tox/issues/1654>`_
- Allow escaping curly braces in setenv. - by :user:`mkenigs`
`1656 <https://github.com/tox-dev/tox/issues/1656>`_


Features
^^^^^^^^

- Support for comments within ``setenv`` and environment files via the ``files|`` prefix. - by :user:`gaborbernat`
`1667 <https://github.com/tox-dev/tox/issues/1667>`_

Links

• PyPI: https://pypi.org/project/tox
• Changelog: https://pyup.io/changelogs/tox/
• Docs: http://tox.readthedocs.org

[pyup-bot]

Closing this in favor of #220