Add .swp fils to gitignore (#2903)

Vim creates .swp files while editing files. These should be included in
the gitignore to ensure that they don't accidentally get committed and
to make it easier for users who wish to utilize commands like git add
--all.

.gitignore

@@ -54,3 +54,4 @@
 venv
 .tox/
 .mypy_cache/
+*.swp