Fix Python 3.8 SyntaxWarning: "is not" with a literal

Fix Python 3.8 SyntaxWarning: "is not" with a literal

### Description

Fixes this warning from Python 3.8 in `Query._set_entities`:

```
lib/sqlalchemy/orm/query.py:179: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if entities is not ():
```

Fixes #4938.

### Checklist

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: #4939
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4939
Pull-request-sha: 88167460ced7f576a9aed3d0ff0a6e7e18810004

Change-Id: I21464c3e69b4db6307db99f9ccb9b34bbac4e744