Feature optimize cache (#4)

* Add code of conduct

* Optimize cache usage

Just a foolproof. There is no need to have infinite
cache.

* Remove outdated code

src/pycountries/currencies.py

@@ -2,34 +2,20 @@
 
 from decimal import Decimal
 from enum import Enum
-from functools import singledispatchmethod
+from functools import lru_cache, singledispatchmethod
 
 from pydantic import Field
 
 from pycountries._base import EnumTypeBase, UnitBase
 
-try:
-    from functools import cache
-except ImportError:
-    from functools import lru_cache
-
-    def cache(  # type: ignore[misc]
-        user_function,
-        /,
-    ):
-        """
-        https://github.com/python/cpython/commit/21cdb711e3b1975398c54141e519ead02670610e
-        """
-        return lru_cache(maxsize=None)(user_function)
-
 
 class CurrencyUnit(UnitBase):
     digits: int = Field(
         default=2,
     )
 
 
-@cache
+@lru_cache(maxsize=3)
 def _get_currencies_by_digits(
     digits: int,
     /,