fix: Correct the AutogenContext.metadata typing to include Sequence[MetaData].

fix: Correct the AutogenContext.metadata typing to include Sequence[MetaData].

### Description
The type annotation for AutogenContext.metadata is currently `Optional[MetaData]`, but `target_metadata` is `Union[MetaData, Sequence[MetaData], None]`. Seems like setting `target_metadata` to `[]` directly translates into the list that `AutogenContext` receives, and that the code is already coercing the potential single/sequence to always be a list.

My alembic plugin wasn't aware that this **could** be a list, and as such wasn't handling the possibility properly.

### 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: #1547
Pull-request: https://github.com/sqlalchemy/alembic/pull/1547
Pull-request-sha: 1bdfa18739e3d156f267953c71267feeded9543b

Change-Id: Ib3114b19c10983114b834676ada69d7475e82fe5