MAINT: fixing compatibility of tox v4 (#161)

Author: bsipocz

tox.ini

@@ -20,6 +20,8 @@ deps =
     # oldestdeps: matplotlib==3.1.2
     # oldestdeps: scipy==1.4
 
+allowlist_externals = bash
+
 commands =
     devdeps: pip install -U --pre --only-binary :all: -i https://pypi.anaconda.org/scipy-wheels-nightly/simple numpy
     devdeps: pip install -U --pre --only-binary :all: -i https://pypi.anaconda.org/scipy-wheels-nightly/simple scipy