workaround for doctest comparison of np dtypes

[NEP51](https://numpy.org/neps/nep-0051-scalar-representation.html#backward-compatibility)
changes the representation of scalars. While the long term strategy
should be a comparison with the new form of representation, the
discussion in [issue
24470](numpy/numpy#24470) seems to suggest to
use an intermediate step while both, numpy-1 and numpy-2 are supported.

navis/morpho/mmetrics.py

@@ -30,6 +30,12 @@
 # Set up logging
 logger = config.get_logger(__name__)
 
+# Set up numpy number representation, see NEP51
+# Once numpy<=2 is dropped from requirements, the doctest comparissons
+# should become `np.float64(1.074)` instead of `1.074`
+if int(np.version.version.split('.')[0])>=2:
+    np.set_printoptions(legacy="1.25")
+
 __all__ = sorted(
     [
         "strahler_index",