chore(internal): exclude mypy from running on tests (#639)

mypy.ini

@@ -5,7 +5,10 @@ show_error_codes = True
 # Exclude _files.py because mypy isn't smart enough to apply
 # the correct type narrowing and as this is an internal module
 # it's fine to just use Pyright.
-exclude = ^(src/lithic/_files\.py|_dev/.*\.py|src/lithic/resources/external_bank_accounts/external_bank_accounts\.py)$
+#
+# We also exclude our `tests` as mypy doesn't always infer
+# types correctly and Pyright will still catch any type errors.
+exclude = ^(src/lithic/_files\.py|_dev/.*\.py|tests/.*|src/lithic/resources/external_bank_accounts/external_bank_accounts\.py)$
 
 strict_equality = True
 implicit_reexport = True