Config(): Fix bool comparison to match types (mypy --strict)

MyPy warning was:

    src/waybar_check_gmail/config/config.py:73:20:73:46: error: Non-overlapping identity check (left operand type: "Union[str, bool, None]", right operand type: "Type[bool]")  [comparison-overlap]
                        if self._map[sec][key] is bool:
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~
    Found 1 error in 1 file (checked 1 source file)

src/waybar_check_gmail/config/config.py

@@ -70,7 +70,7 @@ def __init__(self, fn: AnyPath):
             for key, val in config.items(sec):
                 if key not in self._map[sec]:
                     die("Unexpected config key in section {}: {}".format(sec, key))
-                if self._map[sec][key] is bool:
+                if isinstance(self._map[sec][key], bool):
                     self._map[sec][key] = config.getboolean(sec, key)
                 else:
                     self._map[sec][key] = val