Use URLs instead of IDs for constants

This makes it easier to go from this file to the actual data for the
item or property, either to check that the attribute name in the file is
correct, or to look something up about the entity.

rock_paper_sand/wikidata_value.py

@@ -86,8 +86,10 @@ def uri(self) -> str:
         return f"{_ITEM_PREFIX_CANONICAL_URI}{self.id}"
 
 
-Q_GREGORIAN_CALENDAR = Item("Q12138")
-Q_PROLEPTIC_GREGORIAN_CALENDAR = Item("Q1985727")
+_i = Item.from_string
+Q_GREGORIAN_CALENDAR = _i("https://www.wikidata.org/wiki/Q12138")
+Q_PROLEPTIC_GREGORIAN_CALENDAR = _i("https://www.wikidata.org/wiki/Q1985727")
+del _i
 
 _PROPERTY_PREFIX_FOR_HUMAN = "https://www.wikidata.org/wiki/Property:"
 
@@ -117,7 +119,9 @@ def from_string(cls, value: str) -> Self:
         )
 
 
-P_DATE_OF_FIRST_PERFORMANCE = Property("P1191")
-P_END_TIME = Property("P582")
-P_PUBLICATION_DATE = Property("P577")
-P_START_TIME = Property("P580")
+_p = Property.from_string
+P_DATE_OF_FIRST_PERFORMANCE = _p("https://www.wikidata.org/wiki/Property:P1191")
+P_END_TIME = _p("https://www.wikidata.org/wiki/Property:P582")
+P_PUBLICATION_DATE = _p("https://www.wikidata.org/wiki/Property:P577")
+P_START_TIME = _p("https://www.wikidata.org/wiki/Property:P580")
+del _p