Do not instantiate ClientSession already on import (FlexMeasures#87)

* Do not instantiate ClientSession already on import, outside of doing actual requests

Signed-off-by: Nicolas Höning <nicolas@seita.nl>

* create session after client initialization

Signed-off-by: Nicolas Höning <nicolas@seita.nl>

---------

Signed-off-by: Nicolas Höning <nicolas@seita.nl>

src/flexmeasures_client/client.py

@@ -55,9 +55,11 @@ class FlexMeasuresClient:
     polling_timeout: float = POLLING_TIMEOUT  # seconds
     request_timeout: float = REQUEST_TIMEOUT  # seconds
     polling_interval: float = POLLING_INTERVAL  # seconds
-    session: ClientSession = ClientSession()
+    session: ClientSession | None = None
 
     def __post_init__(self):
+        if self.session is None:
+            self.session = ClientSession()
         if not re.match(r".+\@.+\..+", self.email):
             raise EmailValidationError(
                 f"{self.email} is not an email address format string"