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.

pyproject.toml

@@ -68,9 +68,13 @@ 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
 

src/blacken_docs/__init__.py

@@ -172,7 +172,7 @@ def finish_fragment() -> None:
                     code += f"{PYCON_CONTINUATION_PREFIX}\n"
                 fragment = None
 
-        indentation = None
+        indentation: int | None = None
         for line in match["code"].splitlines():
             orig_line, line = line, line.lstrip()
             if indentation is None and line: