Fix blocking IO in event loop (#374)

Author: joostlek

src/python_opensky/opensky.py

@@ -26,6 +26,9 @@
     from typing_extensions import Self
 
 
+VERSION = metadata.version(__package__)
+
+
 @dataclass
 class OpenSky:
     """Main class for handling connections with OpenSky."""
@@ -97,7 +100,6 @@ async def _request(
             OpenSkyrror: Received an unexpected response from the OpenSky API.
 
         """
-        version = metadata.version(__package__)
         url = URL.build(
             scheme="https",
             host=self.api_host,
@@ -106,7 +108,7 @@ async def _request(
         ).joinpath(uri)
 
         headers = {
-            "User-Agent": f"PythonOpenSky/{version}",
+            "User-Agent": f"PythonOpenSky/{VERSION}",
             "Accept": "application/json, text/plain, */*",
         }