build: remove pip pin

jazzband/pip-tools#1617 was fixed in pip-tools==6.6.1

Author: nedbat

requirements/pins.pip

@@ -7,9 +7,6 @@
 # but have different pins.  This seems to satisfy them all:
 #docutils>=0.17,<0.18
 
-# https://github.com/jazzband/pip-tools/issues/1617
-pip<22.1
-
 # requests gets different versions in dev.pip and doc/requirements.pip, not
 # sure why, and they then ask for different versions of certifi, and we can't
 # install, so pin certifi.