Add allowlist_externals for npm.

By default tox does not allow invoking external binaries, in order to
prevent people from accidentally (or nefariously) deleting their entire
system. The pipeline tests do need to run npm though, so this change
adds that to the configuration.

tox.ini

@@ -58,6 +58,7 @@ ignore_outcome =
     djmain: True
 ignore_errors =
     djmain: True
+allowlist_externals=npm
 
 [testenv:docs]
 basepython = python3.9