MNT: Fix xpass and make xpass fail suite

The fix in pint for the original issue has been released, so we can
have a condition on the xpass.

pyproject.toml

@@ -102,6 +102,7 @@ markers = "xfail_dask: marks tests as expected to fail with Dask arrays"
 norecursedirs = "build docs .idea"
 doctest_optionflags = "NORMALIZE_WHITESPACE"
 mpl-results-path = "test_output"
+xfail_strict = true
 filterwarnings = [
     "ignore:numpy.ndarray size changed:RuntimeWarning",
     # To be removed in the next python-dateutil release.

tests/calc/test_indices.py

@@ -6,6 +6,8 @@
 from datetime import datetime
 
 import numpy as np
+import packaging.version
+import pint
 import pytest
 import xarray as xr
 
@@ -130,7 +132,8 @@ def test_weighted_continuous_average():
     assert_almost_equal(v, 6.900543760612305 * units('m/s'), 7)
 
 
-@pytest.mark.xfail(reason='hgrecco/pint#1593')
+@pytest.mark.xfail(packaging.version.parse(pint.__version__) < packaging.version.parse('0.21'),
+                   reason='hgrecco/pint#1593')
 def test_weighted_continuous_average_temperature():
     """Test pressure-weighted mean temperature function with vertical interpolation."""
     data = get_upper_air_data(datetime(2016, 5, 22, 0), 'DDC')