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, 9 insertions, 9 deletions

diff --git a/alembic/operations/ops.py b/alembic/operations/ops.py
index a40704f..789fb58 100644
--- a/alembic/operations/ops.py
+++ b/alembic/operations/ops.py
@@ -2339,14 +2339,12 @@ class ExecuteSQLOp(MigrateOperation):
             from sqlalchemy import String
             from alembic import op
 
-            account = table('account',
-                column('name', String)
-            )
+            account = table("account", column("name", String))
             op.execute(
-                account.update().\\
-                    where(account.c.name==op.inline_literal('account 1')).\\
-                    values({'name':op.inline_literal('account 2')})
-                    )
+                account.update()
+                .where(account.c.name == op.inline_literal("account 1"))
+                .values({"name": op.inline_literal("account 2")})
+            )
 
         Above, we made use of the SQLAlchemy
         :func:`sqlalchemy.sql.expression.table` and
@@ -2370,11 +2368,13 @@ class ExecuteSQLOp(MigrateOperation):
         also be used normally, use the "bind" available from the context::
 
             from alembic import op
+
             connection = op.get_bind()
 
             connection.execute(
-                account.update().where(account.c.name=='account 1').
-                values({"name": "account 2"})
+                account.update()
+                .where(account.c.name == "account 1")
+                .values({"name": "account 2"})
             )
 
         Additionally, when passing the statement as a plain string, it is first