Replace pkg_resources in test_load (#1870)

Replace pkg_resources.resource_filename with importlib.resources.files.
This removes an implicit dependency on setuptools (to which
pkg_resources belongs); furthermore, the entire pkg_resources API is
deprecated.

Regarding the switch from __file__ to __package__, see:
python/importlib_resources#60

pint/testsuite/test_unit.py

@@ -295,11 +295,11 @@ def test_define(self):
         assert len(dir(ureg)) > 0
 
     def test_load(self):
-        import pkg_resources
+        from importlib.resources import files
 
         from .. import compat
 
-        data = pkg_resources.resource_filename(compat.__name__, "default_en.txt")
+        data = files(compat.__package__).joinpath("default_en.txt")
         ureg1 = UnitRegistry()
         ureg2 = UnitRegistry(data)
         assert dir(ureg1) == dir(ureg2)