codespell: ignore unittest naming

DifferentialOrange · DifferentialOrange · commit 5507a6eb445d · 2024-05-24T10:46:34.000+03:00
After resent update, codespell started to treat assertIn as a typo, and
CI started to fail.
diff --git a/.codespellrc b/.codespellrc
@@ -1,3 +1,3 @@
 [codespell]
 skip = tarantool/msgpack_ext/types/timezones/timezones.py
-ignore-words-list = ans,gost,ro
+ignore-words-list = ans,gost,ro,assertIn
