Reinstate D212 Ruff rule.

This also updated one docstring in cf_units/__init__.py (correctly) due
to the removal of a redundant `noqa: E501` marker in a previous commit

cf_units/__init__.py

@@ -1637,8 +1637,7 @@ def __ne__(self, other):
         return not self == other
 
     def change_calendar(self, calendar):
-        """
-        Returns a new unit with the requested calendar, modifying the
+        """Returns a new unit with the requested calendar, modifying the
         reference date if necessary.  Only works with calendars that
         represent the real world (standard, proleptic_gregorian, julian)
         and with short time intervals (days or less).

pyproject.toml

@@ -208,6 +208,9 @@ ignore = [
 preview = false
 select = [
     "ALL",
+    # Note: the above "all" disables conflicting rules, if you want that
+    # rule it needs to be explicitly enabled below:
+    "D212",  # Multi-line docstring summary should start at the first line
 ]
 
 [tool.ruff.lint.isort]