add overload stubs for proxied classes

add overload stubs for proxied classes

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### Description
Closes #1146
Closes #1147

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Overloaded functions would not have type stubs generated by the stub generator for proxied classes. Now they will.

### 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.

Closes: #1148
Pull-request: https://github.com/sqlalchemy/alembic/pull/1148
Pull-request-sha: ed3c28cc78e57314b7a4e533d77108efc6751949

Change-Id: I7c0ee9d333015174ee6ab754909748f745af2ff9