fix[ci]: pin hexbytes to pre-1.0.0 (vyperlang#3903)

hexbytes introduces new behavior how it interacts with strings/bytes.
this commit pins hexbytes as a stopgap solution until we update tests.

setup.py

@@ -21,6 +21,7 @@
         "hypothesis[lark]>=6.0,<7.0",
         "eth-stdlib==0.2.7",
         "setuptools",
+        "hexbytes<1.0",
         "typing_extensions",  # we can remove this once dependencies upgrade to eth-rlp>=2.0
     ],
     "lint": [