Fix type annotation for `url`

Fix type annotation for `url`

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Left the `pyi` generated file untouched.
### Description
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### Checklist
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This pull request is:

- [x] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [ ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #1209
Pull-request: https://github.com/sqlalchemy/alembic/pull/1209
Pull-request-sha: d15cb6e28d5bb934d139ced87e713a9d77505346

Change-Id: I8548806a693b150f1417a102e6498119524008a0