Add tests to known first parties in isort config (#279)

Author: florimondmanca

setup.cfg

@@ -10,7 +10,7 @@ ignore_missing_imports = True
 combine_as_imports = True
 force_grid_wrap = 0
 include_trailing_comma = True
-known_first_party = httpx
+known_first_party = httpx,tests
 known_third_party = brotli,certifi,chardet,cryptography,h11,h2,hstspreload,nox,pytest,rfc3986,setuptools,trustme,uvicorn
 line_length = 88
 multi_line_output = 3