fix: add missing packaging dependency for feature requiring urllib3 (#1732)

Author: harkamaljot

google/auth/transport/urllib3.py

@@ -34,13 +34,21 @@
 try:
     import urllib3  # type: ignore
     import urllib3.exceptions  # type: ignore
+    from packaging import version  # type: ignore
 except ImportError as caught_exc:  # pragma: NO COVER
     raise ImportError(
-        "The urllib3 library is not installed from please install the "
-        "urllib3 package to use the urllib3 transport."
+        ""
+        f"Error: {caught_exc}."
+        " The 'google-auth' library requires the extras installed "
+        "for urllib3 network transport."
+        "\n"
+        "Please install the necessary dependencies using pip:\n"
+        "  pip install google-auth[urllib3]\n"
+        "\n"
+        "(Note: Using '[urllib3]' ensures the specific dependencies needed for this feature are installed. "
+        "We recommend running this command in your virtual environment.)"
     ) from caught_exc
 
-from packaging import version  # type: ignore
 
 from google.auth import environment_vars
 from google.auth import exceptions
@@ -414,7 +422,7 @@ def urlopen(self, method, url, body=None, headers=None, **kwargs):
                 body=body,
                 headers=headers,
                 _credential_refresh_attempt=_credential_refresh_attempt + 1,
-                **kwargs
+                **kwargs,
             )
 
         return response

setup.py

@@ -34,6 +34,7 @@
     "reauth": "pyu2f>=0.1.5",
     "enterprise_cert": ["cryptography", "pyopenssl"],
     "pyjwt": ["pyjwt>=2.0", "cryptography>=38.0.3"],
+    "urllib3": ["urllib3", "packaging"],
 }
 
 with io.open("README.rst", "r") as fh: