Replace the MD5 usage by SHA256

MD5 is old, insecure, and can create problems for people using this package when they are trying to pass some compliance requirements (for example, FIPS).

locust/stats.py

@@ -702,7 +702,7 @@ def parse_error(cls, error: Optional[Union[Exception, str]]) -> str:
     @classmethod
     def create_key(cls, method: str, name: str, error: Optional[Union[Exception, str]]) -> str:
         key = f"{method}.{name}.{StatsError.parse_error(error)!r}"
-        return hashlib.md5(key.encode("utf-8")).hexdigest()
+        return hashlib.sha256(key.encode("utf-8")).hexdigest()
 
     def occurred(self) -> None:
         self.occurrences += 1