Add type hints to test_base.py (encode#2445)

* added type annotations to test-base.py

* deleted unused imports

* fixed import order

* conditional import

* conditional import TestClient on types.py

* using string literals when importing in types

* added missing imports

* deleted starlette/types, refactored test_base types

* deleted types

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Co-authored-by: Scirlat Danut <[email protected]>
Co-authored-by: Marcelo Trylesinski <[email protected]>