Add clarification about from_string tests limitations

`test_to_and_from_string()` is **extremely** fragile, even the most
innocent change can break it, so we add some extra comments to make sure
other people don't attempt to change it and avoid them some headaches.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

tests/timeseries/test_quantities.py

@@ -599,6 +599,9 @@ def test_invalid_multiplications() -> None:
             quantity *= 200.0  # type: ignore
 
 
+# We can't use _QUANTITY_TYPES here, because it will break the tests, as hypothesis
+# will generate more values, some of which are unsupported by the quantities. See the
+# test comment for more details.
 @pytest.mark.parametrize("quantity_type", [Power, Voltage, Current, Energy, Frequency])
 @pytest.mark.parametrize("exponent", [0, 3, 6, 9])
 @hypothesis.settings(
@@ -623,8 +626,14 @@ def test_to_and_from_string(
     generation and regenerate it with the more appropriate unit and precision.
     """
     quantity = quantity_type.__new__(quantity_type)
-    # pylint: disable=protected-access
-    quantity._base_value = value * 10**exponent
+    quantity._base_value = value * 10**exponent  # pylint: disable=protected-access
+    # The above should be replaced with:
+    # quantity = quantity_type._new(  # pylint: disable=protected-access
+    #     value, exponent=10**exponent
+    # )
+    # But we can't do that now, because, you guessed it, it will also break the tests
+    # (_new() will use 10.0**exponent instead of 10**exponent, which seems to have some
+    # effect on the tests.
     quantity_str = f"{quantity:.{exponent}}"
     from_string = quantity_type.from_string(quantity_str)
     try: