Update tox configuration for PEP 517

This drops "usedevelop", which does an editable installation of the
directory in favor of an isolated PEP 517/518 build environment. The
minimum version of tox has been bumped to one that has PEP 518 support.

examples/rustapi_module/tox.ini

@@ -3,11 +3,11 @@ envlist = py35,
           py36,
           py37,
           pypy35
-minversion = 2.9.0
+minversion = 3.4.0
 skip_missing_interpreters = true
+isolated_build = true
 
 [testenv]
 description = Run the unit tests under {basepython}
 deps = -rrequirements-dev.txt
-usedevelop = True
 commands = pytest {posargs}