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

1 files changed, 2 insertions, 0 deletions

diff --git a/setup.cfg b/setup.cfg
index 39c1540..a7666ce 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -89,10 +89,12 @@ ignore =
     RST304,RST303,RST299,RST399,
     W503,W504
 exclude = .venv,.git,.tox,dist,doc,*egg,build
+filename = *.py,*.pyi
 import-order-style = google
 application-import-names = alembic,tests
 per-file-ignores =
                 **/__init__.py:F401
+                **/*.pyi:E302,E704,E266
 max-line-length = 79
 
 [sqla_testing]