fix[ci]: fix README encoding in setup.py (vyperlang#4348)

the call to `setup()` would fail on windows when there are unicode
characters in `README.md`, because files are apparently opened with
encoding `cp1252` by default on windows. this commit ensures the file
is opened with `utf-8` encoding.

setup.py

@@ -36,7 +36,7 @@
 
 extras_require["dev"] = extras_require["dev"] + extras_require["test"] + extras_require["lint"]
 
-with open("README.md", "r") as f:
+with open("README.md", "r", encoding="utf-8") as f:
     long_description = f.read()