Improve MyPy configuration

Some improvments flagged by Repo-Review: https://learn.scientific-python.org/development/guides/repo-review/

1. Remove now-default show_error_codes.
2. Enable some optional error codes with extra checks.

Author: adamchainz

pyproject.toml

@@ -77,9 +77,13 @@ source = [
 show_missing = true
 
 [tool.mypy]
+enable_error_code = [
+  "ignore-without-code",
+  "redundant-expr",
+  "truthy-bool",
+]
 mypy_path = "src/"
 namespace_packages = false
-show_error_codes = true
 strict = true
 warn_unreachable = true