Failing tests after pint fix, with pint 0.20.1

Getting locally failing unit tests since the pint fix in !282 (merged), with pint v0.20.1.

The dependency is pint >=0.20, !=0.24.0, so my local version should be allowed.

@muellese Any idea?

    def test_cf_units(self):
        self.assertEqual(u("mm"), "mm")
        self.assertEqual(u("millimeter"), "mm")
        self.assertEqual(u("kilometer"), "km")

        self.assertEqual(u("meter ** 2"), "m2")
        self.assertEqual(u("1 / meter"), "m-1")

        self.assertEqual(u("degree_north"), "degrees_north")
        self.assertEqual(u("degree_east"), "degrees_east")

        self.assertEqual(u("°C"), "°C")
        self.assertEqual(u("degC"), "°C")
        self.assertEqual(u("degree_Celsius"), "°C")

>       self.assertEqual(u(""), "1")
E       AssertionError: 'dimensionless' != '1'
E       - dimensionless
E       + 1

tests\data\test_cf_units.py:26: AssertionError