Use pytest.hookimpl in pytest_configure (#49)

This patch moves away from using the deprecated mechanism in order to clear out the warning in pytest 7.

Resolves #47.

pytest_aiohttp/plugin.py

@@ -18,7 +18,7 @@
 )
 
 
-@pytest.mark.tryfirst
+@pytest.hookimpl(tryfirst=True)
 def pytest_configure(config) -> None:
     val = config.getoption("asyncio_mode")
     if val is None: