Fix cookieError on python<3.8

Only occurs in combination with api-server enabled,
due to some hot-fixing starlette does.
Since we load starlette at a later point, we need to replicate
starlette's behaviour for now, so sameSite cookies don't create a
problem.

closes #4356

freqtrade/exchange/exchange.py

@@ -3,6 +3,7 @@
 Cryptocurrency Exchanges support
 """
 import asyncio
+import http
 import inspect
 import logging
 from copy import deepcopy
@@ -34,6 +35,12 @@ CcxtModuleType = Any
 logger = logging.getLogger(__name__)
 
 
+# Workaround for adding samesite support to pre 3.8 python
+# Only applies to python3.7, and only on certain exchanges (kraken)
+# Replicates the fix from starlette (which is actually causing this problem)
+http.cookies.Morsel._reserved["samesite"] = "SameSite"  # type: ignore
+
+
 class Exchange:
 
     _config: Dict = {}