TST: skip doctests for py39

tox.ini

@@ -20,7 +20,7 @@ depends = begin
 deps =
     pytest
     h5py
-    pint
+    !py39: pint
     !py39: astropy!=6.1.1
     coverage[toml]>=5.0
     pytest-cov
@@ -32,6 +32,14 @@ commands =
     pytest --cov=unyt --cov-append --doctest-modules --doctest-plus --doctest-rst --basetemp={envtmpdir}
     coverage report --omit='.tox/*'
 
+[testenv:py39]
+# skip doctest on py39 because doctests require numpy>=2.0 and all optional deps,
+# but some of our optional deps (pint, astropy) don't have a version that support
+# both numpy>=2.0 and Python 3.9
+commands=
+    pytest --cov=unyt --cov-append --basetemp={envtmpdir}
+    coverage report --omit='.tox/*'
+
 [testenv:py39-versions]
 deps =
     docutils