Update virtualenv to 20.0.15

[pyup-bot]

This PR updates virtualenv from 20.0.13 to 20.0.15.

Changelog

20.0.14

~~~~~~~~~~~~~~~~~~
- Fix discovery of interpreter by name from ``PATH`` that does not match a spec format - by :user:`gaborbernat`. (`1746 <https://github.com/pypa/virtualenv/issues/1746>`_)

Links

• PyPI: https://pypi.org/project/virtualenv
• Changelog: https://pyup.io/changelogs/virtualenv/
• Homepage: https://virtualenv.pypa.io/

[codecov-io]

Codecov Report

Merging #1095 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1095   +/-   ##
=======================================
  Coverage   94.02%   94.02%           
=======================================
  Files          10       10           
  Lines         787      787           
=======================================
  Hits          740      740           
  Misses         47       47           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d1901d3...37fa0ef. Read the comment docs.

[pyup-bot]

Closing this in favor of #1102