Detection of PEP 604 union syntax.

Detection of PEP 604 union syntax.

### Description

Fixes #8478

Handle `UnionType` as arguments to `Mapped`, e.g., `Mapped[str | None]`:

- adds `utils.typing.is_optional_union()` used to detect if a column should be nullable.
- adds `"UnionType"` to `utils.typing.is_optional()` names.
- uses `get_origin()` in `utils.typing.is_origin_of()` as `UnionType` has no `__origin__` attribute.
- tests with runtime type and postponed annotations and guard the tests running with `compat.py310`.

### 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: #8479
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/8479
Pull-request-sha: 12417654822272c5847e684c53677f665553ef0e

Change-Id: Ib3248043dd4a97324ac592c048385006536b2d49