allow tuple type in down_revision in migration templates

allow tuple type in down_revision in migration templates

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FIXES #1665

Currently the type of `down_revision` in migration templates is `Union[str, None]`. However, when you generate a migration using `alembic merge heads` it will generate a migration with a tuple of strings in `down_revision` which causes a type error.

### Description

f.e. when you have 2 heads and run `alembic merge heads` you will get a migration that looks like this:

```py
revision: str = "6cee9e23ed4b"
down_revision: Union[str, None] = ("aba1172f1a9f", "cdd5e67db84f")
branch_labels: Union[str, Sequence[str], None] = None
depends_on: Union[str, Sequence[str], None] = None
```

which will give a type error with type checkers like mypy and pyright. This change fixes this. From the code it seems like just like `branch_labels` and `depends_on` the `down_revision` is first casted to a tuple if needed so typing it as `Sequence[str]` and not `Tuple[str, ...]` should be safe.

### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [x] 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: #1666
Pull-request: https://github.com/sqlalchemy/alembic/pull/1666
Pull-request-sha: 186df96240a621ba71247a4f5cbd6413e3bfc270

Change-Id: I7b203b85bf69f215871995e4c4f5b96c20528c7d