Upgrade requests to support a more recent idna

This is the correct requests version according to:
psf/requests#4890 (comment)

setup.py

@@ -33,11 +33,8 @@
         "cachetools>=3.1.0,<4.0.0",
         "coincurve>=10.0.0,<11.0.0",
         "eth-utils>=1.8.4,<2",
-        # Fixing this dependency due to: requests 2.20.1 has requirement
-        # idna<2.8,>=2.5, but you'll have idna 2.8 which is incompatible.
-        "idna==2.7",
-        # idna 2.7 is not supported by requests 2.18
-        "requests>=2.20,<3",
+        # requests 2.21 is required to support idna 2.8 which is required elsewhere
+        "requests>=2.21,<3",
         "ipython>=7.8.0,<7.10.0",  # attach fails with v7.10.{0,1}
         "plyvel==1.2.0",
         PYEVM_DEPENDENCY,