collect python_implementation in easypost-python user-agent

Author: Roguelazer

easypost/requestor.py

@@ -60,6 +60,7 @@ def request_raw(self, method, url, params=None, api_key_required=True):
             ("lang_version", platform.python_version),
             ("platform", platform.platform),
             ("uname", lambda: " ".join(platform.uname())),
+            ("implementation", platform.python_implementation),
         ):
             try:
                 val = func()