Upgrade virtualenv dependency to 20.x

[florimondmanca]

What does this PR do?

Pin dependency on virtualenv to 20.*.

Motivation

• virtualenv 20.0.0 is officially out of beta: https://twitter.com/gjbernat/status/1226803593535279104
• Follow-up to Use the latest beta release of virtualenv for performance improvements #5617

Additional Notes

Validated on a fresh install:

# Upgrade ddev, which installs new `virtualenv`.
pip install -U -e "./datadog_checks_dev[cli]"

# Nuke existing virtualenvs in an integration (pick any)
rm -r http_check/.tox

# Run tests, which will recreate venvs with `virtualenv` 20.x.
ddev test http_check

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #5680 into master will decrease coverage by 4.72%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
datadog_checks_dev/datadog_checks/dev/__main__.py	100.00% <0.00%> (ø)	⬆️
datadog_checks_dev/tests/common.py	100.00% <0.00%> (ø)	⬆️
datadog_checks_dev/tests/conftest.py	100.00% <0.00%> (ø)	⬆️
datadog_checks_dev/tests/test__env.py	100.00% <0.00%> (ø)	⬆️
datadog_checks_dev/tests/test_docker.py	100.00% <0.00%> (ø)	⬆️
datadog_checks_dev/tests/test_conditions.py	100.00% <0.00%> (ø)	⬆️
datadog_checks_dev/tests/test_structures.py	100.00% <0.00%> (ø)	⬆️
datadog_checks_dev/tests/test_subprocess.py	100.00% <0.00%> (ø)	⬆️
datadog_checks_dev/datadog_checks/dev/env.py	43.18% <0.00%> (ø)	⬆️
datadog_checks_dev/tests/tooling/test_git.py	100.00% <0.00%> (ø)	⬆️
... and 889 more

[florimondmanca]

Note: some users are experiencing issues with v20: pypa/virtualenv#1551.

But it looks like we're not affected. (Validated that virtualenv creation and tests run fine on a fresh install -- see updated PR description. Also the PR Changes job here successfully got Tox to create a venv for 2.7 and 3.8.)