dev-requirements: install vcrpy from a fork without urllib3 version cap

This should be more or less fine, since only CI and maintainers are
likely to bother installing the dev-requirements. Packaged releases
don't include these, so we can move pretty quickly to get current again
whenever the vcrpy maintainers accept the change (or if they do).

Chose to create a fork within Sopel's GitHub organization specifically
to avoid any accidental breakage from deleting my personal fork, which
might happen "on autopilot" if/when the PR is accepted. (I habitually
delete forks from which I don't plan to make further contributions after
whatever patch I submitted is merged.)

dev-requirements.txt

@@ -16,7 +16,9 @@ sphinx>=4,<5
 sphinxcontrib-autoprogram==0.1.8
 # custom plugin to help with RFC links
 sphinx-rfcsection~=0.1.1
-vcrpy>=5,<6
+# use fork of vcrpy 5.x until kevin1024/vcrpy#777 is (hopefully) accepted
+# (or until py3.9 EOL... in 10/2025, I HOPE NOT)
+vcrpy @ git+https://github.com/sopel-irc/vcrpy@uncap-urllib3
 # type check
 mypy>=1.3,<2
 sqlalchemy[mypy]>=1.4,<1.5