Bump tox from 3.20.1 to 3.21.0

[dependabot]

Bumps tox from 3.20.1 to 3.21.0.

Changelog

Sourced from tox's changelog.

v3.21.0 (2021-01-08)

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) <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) <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) <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) <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) <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) <https://github.com/tox-dev/tox/issues/1502>_
• Allow {posargs} in setenv. - by :user:jayvdb [#1695](https://github.com/tox-dev/tox/issues/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) <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) <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) <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) <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) <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) <https://github.com/tox-dev/tox/issues/1534>_

Commits

• 2cc64c1 release 3.21.0
• 49edaed Add instructions for choosing pip version (#1771)
• 91e180d Fix CI (#1774)
• 6f5e2e9 updated poetry build system (#1770)
• 17fefb1 [pre-commit.ci] pre-commit autoupdate (#1769)
• 4061f56 Fix download=True option (#1558) (#1766)
• 65641d4 Remove high constraint for importlib-metadata (#1764)
• 58ee845 Correct numbering in System Overview section (#1767)
• 69af6cd Add py39 test environment to docs (#1756)
• f04c16f Doc: Don't use deprecated plugin in the example (#1757)
• Additional commits viewable in compare view

Dependabot will resolve any conflicts with this PR as long as you don't alter it yourself. You can also trigger a rebase manually by commenting @dependabot rebase.

---

Dependabot commands and options

You can trigger Dependabot actions by commenting on this PR:

• @dependabot rebase will rebase this PR
• @dependabot recreate will recreate this PR, overwriting any edits that have been made to it
• @dependabot merge will merge this PR after your CI passes on it
• @dependabot squash and merge will squash and merge this PR after your CI passes on it
• @dependabot cancel merge will cancel a previously requested merge and block automerging
• @dependabot reopen will reopen this PR if it is closed
• @dependabot close will close this PR and stop Dependabot recreating it. You can achieve the same result by closing it manually
• @dependabot ignore this major version will close this PR and stop Dependabot creating any more for this major version (unless you reopen the PR or upgrade to it yourself)
• @dependabot ignore this minor version will close this PR and stop Dependabot creating any more for this minor version (unless you reopen the PR or upgrade to it yourself)
• @dependabot ignore this dependency will close this PR and stop Dependabot creating any more for this dependency (unless you reopen the PR or upgrade to it yourself)