fix(tox): force new version of pip to fix infinite installation time

Without this line pip 20.x is using with its new backtracking algo and it takes
more than 12 hours to run.
7 jobs for !86 with topic/default/force-new-version-of-pip-to-fix-infinite-installation-time in 7 minutes and 6 seconds (queued for 4 seconds)
latest detached
Status Name Job ID Coverage
  Lint
passed black #601185
lint black

00:01:16

passed check-manifest #601186
lint check-manifest

00:00:57

passed mypy #601187
lint mypy

00:02:47

 
  Tests
passed py3 #601188
tests py3

00:04:20

 
  Release
passed image_build #601288
release kaniko

00:02:38

failed image_build #601223
release kaniko

00:01:11

failed image_build #601189
release kaniko

00:00:54