devel/py-virtualenv: Update to 21.6.1

ChangeLog:

https://github.com/pypa/virtualenv/releases/tag/21.2.4
https://github.com/pypa/virtualenv/releases/tag/21.3.0
https://github.com/pypa/virtualenv/releases/tag/21.3.1
https://github.com/pypa/virtualenv/releases/tag/21.3.2
https://github.com/pypa/virtualenv/releases/tag/21.3.3
https://github.com/pypa/virtualenv/releases/tag/21.4.0
https://github.com/pypa/virtualenv/releases/tag/21.4.1
https://github.com/pypa/virtualenv/releases/tag/21.4.2
https://github.com/pypa/virtualenv/releases/tag/21.4.3
https://github.com/pypa/virtualenv/releases/tag/21.5.0
https://github.com/pypa/virtualenv/releases/tag/21.5.1
https://github.com/pypa/virtualenv/releases/tag/21.5.2
https://github.com/pypa/virtualenv/releases/tag/21.6.0
https://github.com/pypa/virtualenv/releases/tag/21.6.1

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