Skip the doctests in metrics.py

Otherwise we see the following when we run tests
```
circuit_knitting/utils/metrics.py::circuit_knitting.utils.metrics.cross_entropy
  <doctest circuit_knitting.utils.metrics.cross_entropy[0]>:1: DeprecationWarning: The function ``circuit_knitting.utils.metrics.cross_entropy()`` is deprecated as of circuit-knitting-toolbox 0.7.0. It will be removed Circuit knitting toolbox 0.8.0 release.

circuit_knitting/utils/metrics.py::circuit_knitting.utils.metrics.MSE
  <doctest circuit_knitting.utils.metrics.MSE[0]>:1: DeprecationWarning: The function ``circuit_knitting.utils.metrics.MSE()`` is deprecated as of circuit-knitting-toolbox 0.7.0. It will be removed Circuit knitting toolbox 0.8.0 release.

circuit_knitting/utils/metrics.py::circuit_knitting.utils.metrics.MAPE
  <doctest circuit_knitting.utils.metrics.MAPE[0]>:1: DeprecationWarning: The function ``circuit_knitting.utils.metrics.MAPE()`` is deprecated as of circuit-knitting-toolbox 0.7.0. It will be removed Circuit knitting toolbox 0.8.0 release.

circuit_knitting/utils/metrics.py::circuit_knitting.utils.metrics.chi2_distance
  <doctest circuit_knitting.utils.metrics.chi2_distance[0]>:1: DeprecationWarning: The function ``circuit_knitting.utils.metrics.chi2_distance()`` is deprecated as of circuit-knitting-toolbox 0.7.0. It will be removed Circuit knitting toolbox 0.8.0 release.

circuit_knitting/utils/metrics.py::circuit_knitting.utils.metrics.chi2_distance
  <doctest circuit_knitting.utils.metrics.chi2_distance[1]>:1: DeprecationWarning: The function ``circuit_knitting.utils.metrics.chi2_distance()`` is deprecated as of circuit-knitting-toolbox 0.7.0. It will be removed Circuit knitting toolbox 0.8.0 release.

circuit_knitting/utils/metrics.py::circuit_knitting.utils.metrics.HOP
  <doctest circuit_knitting.utils.metrics.HOP[0]>:1: DeprecationWarning: The function ``circuit_knitting.utils.metrics.HOP()`` is deprecated as of circuit-knitting-toolbox 0.7.0. It will be removed Circuit knitting toolbox 0.8.0 release.
```

tox.ini

@@ -8,7 +8,7 @@ extras =
   test
   cplex
 commands =
-  pytest {posargs}
+  pytest {posargs} --ignore=circuit_knitting/utils/metrics.py
 
 [testenv:style]
 extras =