Use session and remove inline imports in remote_config

Per AI code review feedback:

- Use _session.post() instead of requests.post() to maintain retry logic
- Remove inline imports (datetime, dateutil.tz already imported at top)
- Ensures consistency with other request functions in the module

Addresses code review feedback about maintaining session consistency.

Author: haacked

posthog/request.py

@@ -139,10 +139,6 @@ def remote_config(
     timeout: int = 15,
 ) -> Any:
     """Get remote config flag value from remote_config API endpoint"""
-    import requests
-    from datetime import datetime
-    from dateutil.tz import tzutc
-    
     url = remove_trailing_slash(host or DEFAULT_HOST) + f"/api/projects/@current/feature_flags/{key}/remote_config/"
 
     body = {
@@ -158,7 +154,7 @@ def remote_config(
 
     data = json.dumps(body, cls=DatetimeSerializer)
 
-    res = requests.post(url, data=data, headers=headers, timeout=timeout)
+    res = _session.post(url, data=data, headers=headers, timeout=timeout)
 
     return _process_response(
         res,