fix: add PYTHON_VERSION_STATUS_UNSPECIFIED enum value

Author: vchudnov-g

google/api_core/_python_version_support.py

@@ -25,6 +25,7 @@
 class PythonVersionStatus(enum.Enum):
     """Represent the support status of a Python version."""
 
+    PYTHON_VERSION_STATUS_UNSPECIFIED = "PYTHON_VERSION_STATUS_UNSPECIFIED"
     PYTHON_VERSION_UNSUPPORTED = "PYTHON_VERSION_UNSUPPORTED"
     PYTHON_VERSION_EOL = "PYTHON_VERSION_EOL"
     PYTHON_VERSION_DEPRECATED = "PYTHON_VERSION_DEPRECATED"