fix: enforce constraints during unit tests (#760)

Drop explicit pin / constraint on 'urllib3':  specific 'requests'
versions have very narrow pins, and ours is only likely to create
conflicts.

Bump the 'requests' lower bound to '2.20.0', the lowest version for
which our tests pass once constraints are being checked.

Closes #759
diff --git a/setup.py b/setup.py
index f2fd4e5..a9bfc97 100644
--- a/setup.py
+++ b/setup.py
@@ -33,8 +33,7 @@
 extras = {
     "aiohttp": [
         "aiohttp >= 3.6.2, < 4.0.0dev; python_version>='3.6'",
-        "requests >= 2.18.0, < 3.0.0dev",
-        "urllib3 >= 1.0.0, <2.0.0dev",
+        "requests >= 2.20.0, < 3.0.0dev",
     ],
     "pyopenssl": "pyopenssl>=20.0.0",
     "reauth": "pyu2f>=0.1.5",