Apply lots of refactoring to the websocket subpackage (fix every mypy error and warning). Add integers.py and decimal.py to bfxapi.utils package. Update requirements.txt and setup.py with new mypy dependencies.

bfxapi/utils/decimal.py

@@ -0,0 +1,9 @@
+import json
+
+from decimal import Decimal
+
+class DecimalEncoder(json.JSONEncoder):
+    def default(self, obj):
+        if isinstance(obj, Decimal):
+            return str(obj)
+        return json.JSONEncoder.default(self, obj)