Limit virtualenv version for EOL Python versions

Also, requirements for Python 3.6 is locked and won't be updated anymore.

ref: https://tox.wiki/en/4.11.3/faq.html#testing-end-of-life-python-versions

tox.ini

@@ -1,6 +1,8 @@
 [tox]
 envlist = update, compile, autopep8, docformatter, isort, pylint, flake8, pydocstyle, docs, py{26, 27, 34, 35, 36, 37, 38, 39, 310, 311, 312}
 skip_missing_interpreters = true
+# https://tox.wiki/en/4.11.3/faq.html#testing-end-of-life-python-versions
+requires = virtualenv<20.22.0
 
 [base]
 pip_compile_command = pip-compile --resolver=backtracking --strip-extras --no-emit-index-url --allow-unsafe --upgrade
@@ -89,11 +91,6 @@ basepython = python3.7
 deps = pip-tools
 commands = {[base]pip_compile_command} requirements-tests37.in -o requirements-tests37.txt
 
-[testenv:update_requirements36]
-basepython = python3.6
-deps = pip-tools
-commands = {[base]pip_compile_command} requirements-tests37.in -o requirements-tests36.txt
-
 [testenv:py38]
 deps = -r requirements-tests37.txt