fix: exclude async-specific code from py37 coverage

The AsgiMiddleware class and set_execution_context_async function in
execution_id.py require Python 3.8+ due to async dependencies. These
are now excluded from coverage calculations in Python 3.7 environments.

Author: taeold

.coveragerc-py37

@@ -18,4 +18,9 @@ exclude_lines =
     # Don't complain about async-specific imports and code
     from functions_framework.aio import
     from functions_framework._http.asgi import
-    from functions_framework._http.gunicorn import UvicornApplication
+    from functions_framework._http.gunicorn import UvicornApplication
+    
+    # Exclude async-specific classes and functions in execution_id.py
+    class AsgiMiddleware:
+    async def __call__
+    def set_execution_context_async