Make tox use 'setup.py develop'

Comments inline.

[skip ci]

tox.ini

@@ -13,6 +13,16 @@ whitelist_externals =
     *
 commands =
     make toxtest
+# our Makefile assumes it's being run from the directory
+# containing setup.py. However, that directory also has the "gevent"
+# directory in it. Which means it's on sys.path first. So unless it
+# has the correct binary extensions for the python version/impl we're trying
+# to run, nothing works. By setting usedevelop=True, then the .so gets built
+# in place in that gevent directory, which makes the tox commands run. This
+# can step on the toes of various implementations when they share the same name
+# for files, but at least it lets some implementations co-exist.
+# The real solution is probably to put "gevent" beneath a "src" directory.
+usedevelop = True
 
 [testenv:py27-full]
 basepython = python2.7