Apply repo-review MyPy suggestions

MY103: MyPy warn unreachable

Must have `warn_unreachable` (true/false) to pass this check. There are
occasionally false positives (often due to platform or Python version
static checks), so it's okay to set it to false if you need to. But try
it first - it can catch real bugs too.

MY104: MyPy enables ignore-without-code

Must have `"ignore-without-code"` in `enable_error_code = [...]`. This
will force all skips in your project to include the error code, which
makes them more readable, and avoids skipping something unintended.

MY105: MyPy enables redundant-expr

Must have `"redundant-expr"` in `enable_error_code = [...]`. This helps
catch useless lines of code, like checking the same condition twice.

MY106: MyPy enables truthy-bool

Must have `"truthy-bool"` in `enable_error_code = []`. This catches
mistakes in using a value as truthy if it cannot be falsy.

pyproject.toml

@@ -119,6 +119,10 @@ title_format = "## [{version}](https://github.com/pypa/pipx/tree/{version}) - {p
 issue_format = "[#{issue}](https://github.com/pypa/pipx/issues/{issue})"
 package = "pipx"
 
+[tool.mypy]
+warn_unreachable = true
+enable_error_code = [ "ignore-without-code", "redundant-expr", "truthy-bool" ]
+
 [[tool.mypy.overrides]]
 module = [
   "pycowsay.*",