chore: install a safe pip version

Author: Midnighter

tox.ini

@@ -10,13 +10,19 @@ python =
  3.9: py39
 
 [testenv]
+# Always download the latest pip version. This has not been released yet thus the below
+# environment variable VIRTUALENV_PIP is needed. See
+# https://github.com/tox-dev/tox/issues/1768#issuecomment-787075584
+download = true
 deps =
  pytest
  pytest-cov
  pytest-mock
  pytest-raises
 passenv =
  STRUCTURIZR_*
+setenv =
+ VIRTUALENV_PIP=21.1
 commands =
  pytest --cov=structurizr --cov-report=term {posargs}