tooling: write_pyi.py: filter usage of raw-strings (rstrings)

tooling: write_pyi.py: filter usage of raw-strings (rstrings)

### Description
While reading the diff between [`rel_1_10_2...rel_1_10_3`](https://github.com/sqlalchemy/alembic/compare/rel_1_10_2...rel_1_10_3), the introduction of r-strings drew my attention, and that resulted in some [discussion on the relevant commit](https://github.com/sqlalchemy/alembic/commit/bc0c305b7c2cc0401e250fcd6a725aacecdd6e33).

This changeset filters the production of r-strings during stub-generation to cases where docstrings contain escape (backslash, `\`) characters.

I'll admit that I didn't realize until today that these stubs are primarily for typechecking.  Since that's the case, I have doubts about whether the change is worthwhile (consistency and simplicity -- using r-strings for all docstrings in the stubs -- even if it's redundant, seems fine to me).

### Checklist
This pull request is:

- [x] A documentation / typographical error fix
- [x] A short code fix
	- Relates to discussion at https://github.com/sqlalchemy/alembic/commit/bc0c305b7c2cc0401e250fcd6a725aacecdd6e33

Closes: #1218
Pull-request: https://github.com/sqlalchemy/alembic/pull/1218
Pull-request-sha: 352ab8829525435f78838687e12156a3fa3a3d86

Closes: #1219
Pull-request: https://github.com/sqlalchemy/alembic/pull/1219
Change-Id: I2808a592681dabc093d538f589e673fcc5e05822