Use _session rather than requests

Benefits:

1. Reuses TCP connections via keep-alive
2. 2 retries on connect/read errors
3. Faster handshakes

Author: haacked

posthog/request.py

@@ -194,7 +194,7 @@ def get(
     if etag:
         headers["If-None-Match"] = etag
 
-    res = requests.get(full_url, headers=headers, timeout=timeout)
+    res = _session.get(full_url, headers=headers, timeout=timeout)
 
     # Handle 304 Not Modified
     if res.status_code == 304: