Bump virtualenv from 20.2.2 to 20.4.0 in /requirements

[dependabot]

Bumps virtualenv from 20.2.2 to 20.4.0.

Changelog

Sourced from virtualenv's changelog.

v20.4.0 (2021-01-19)

Features - 20.4.0

- On the programmatic API allow passing in the environment variable dictionary to use, defaults to ``os.environ`` if not
  specified - by :user:`gaborbernat`. (`[#2054](https://github.com/pypa/virtualenv/issues/2054) <https://github.com/pypa/virtualenv/issues/2054>`_)
Bugfixes - 20.4.0

• Upgrade embedded setuptools to 51.3.3 from 51.1.2 - by :user:gaborbernat. ([#2055](https://github.com/pypa/virtualenv/issues/2055) <https://github.com/pypa/virtualenv/issues/2055>_)

v20.3.1 (2021-01-13)

Bugfixes - 20.3.1

- Bump embed pip to ``20.3.3``, setuptools to ``51.1.1`` and wheel to ``0.36.2`` - by :user:`gaborbernat`. (`[#2036](https://github.com/pypa/virtualenv/issues/2036) <https://github.com/pypa/virtualenv/issues/2036>`_)
- Allow unfunctioning of pydoc to fail freely so that virtualenvs can be
  activated under Zsh with set -e (since otherwise ``unset -f`` and
  ``unfunction`` exit with 1 if the function does not exist in Zsh) - by
  :user:`d125q`. (`[#2049](https://github.com/pypa/virtualenv/issues/2049) <https://github.com/pypa/virtualenv/issues/2049>`_)
- Drop cached python information if the system executable is no longer present (for example when the executable is a
  shim and the mapped executable is replaced - such is the case with pyenv) - by :user:`gaborbernat`. (`[#2050](https://github.com/pypa/virtualenv/issues/2050) <https://github.com/pypa/virtualenv/issues/2050>`_)
v20.3.0 (2021-01-10)
Features - 20.3.0

• The builtin discovery takes now a --try-first-with argument and is first attempted as valid interpreters. One can use this to force discovery of a given python executable when the discovery order/mechanism raises errors - by :user:gaborbernat. ([#2046](https://github.com/pypa/virtualenv/issues/2046) <https://github.com/pypa/virtualenv/issues/2046>_)

Bugfixes - 20.3.0

- On Windows python ``3.7+`` distributions where the exe shim is missing fallback to the old ways - by :user:`gaborbernat`. (`[#1986](https://github.com/pypa/virtualenv/issues/1986) <https://github.com/pypa/virtualenv/issues/1986>`_)
- When discovering interpreters on Windows, via the PEP-514, prefer ``PythonCore`` releases over other ones. virtualenv
  is used via pip mostly by this distribution, so prefer it over other such as conda - by :user:`gaborbernat`. (`[#2046](https://github.com/pypa/virtualenv/issues/2046) <https://github.com/pypa/virtualenv/issues/2046>`_)

Commits

• d47e0a5 release 20.4.0
• a9b0f3b Fix Windows failure (#2056)
• 40aec48 API: Allow passing on the environment variable as an argument (#2054)
• 4c95506 Upgrade embedded setuptools to 51.3.3 from 51.1.2 (#2055)
• 7185e57 Merge pull request #2051 from pypa/release-20.3.1
• 395696d release 20.3.1
• 7f83442 Clear stash if system executable is missing (#2050)
• 39fdf95 Bump pip to 20.3.3 and wheel to 0.36.2 (#2036)
• 26943f6 Allow unfunctioing pydoc to fail (#2049)
• a4025d6 release 20.3.0
• Additional commits viewable in compare view

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[dependabot]

Superseded by #558.