Replace pkg_resources in test_load

Replace pkg_resources.resource_filename with
importlib.resources.as_file. 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

@@ -293,16 +293,16 @@ def test_define(self):
         assert len(dir(ureg)) > 0
 
     def test_load(self):
-        import pkg_resources
+        from importlib.resources import as_file, files
 
         from .. import compat
 
-        data = pkg_resources.resource_filename(compat.__name__, "default_en.txt")
-        ureg1 = UnitRegistry()
-        ureg2 = UnitRegistry(data)
-        assert dir(ureg1) == dir(ureg2)
-        with pytest.raises(FileNotFoundError):
-            UnitRegistry(None).load_definitions("notexisting")
+        with as_file(files(compat.__package__).joinpath("default_en.txt")) as data:
+            ureg1 = UnitRegistry()
+            ureg2 = UnitRegistry(data)
+            assert dir(ureg1) == dir(ureg2)
+            with pytest.raises(FileNotFoundError):
+                UnitRegistry(None).load_definitions("notexisting")
 
     def test_default_format(self):
         ureg = UnitRegistry()