Guard type aliases and remove redundant imports in the postgres hook

[Dev-iL]

This attempts to fix the below issue with doc generation in the google provider observed in CI that happens while importing the postgres provider.

Traceback (mostrecentcalllast):
File"/usr/python/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line143, inimport_modulereturnimportlib.import_module(modname)
File"/usr/python/lib/python3.10/importlib/__init__.py", line126, inimport_modulereturn_bootstrap._gcd_import(name, package, level)
File"<frozen importlib._bootstrap>", line1050, in_gcd_importFile"<frozen importlib._bootstrap>", line1027, in_find_and_loadFile"<frozen importlib._bootstrap>", line1006, in_find_and_load_unlockedFile"<frozen importlib._bootstrap>", line688, in_load_unlockedFile"<frozen importlib._bootstrap_external>", line883, inexec_moduleFile"<frozen importlib._bootstrap>", line241, in_call_with_frames_removedFile"/opt/airflow/providers/google/src/airflow/providers/google/cloud/transfers/bigquery_to_postgres.py", line31, in<module>fromairflow.providers.postgres.hooks.postgresimportPostgresHookFile"/opt/airflow/providers/postgres/src/airflow/providers/postgres/hooks/postgres.py", line68, in<module>CursorType: TypeAlias=DictCursor|RealDictCursor|NamedTupleCursorTypeError: unsupportedoperand type(s) for|: 'DictCursor'and'RealDictCursor'

The fix is moving the two TypeAlias definitions into the existing TYPE_CHECKING block. Also, imports were cleaned up a bit.

related: #61532

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[bugraoz93]

CI looks green, thanks @Dev-iL !