FIX: install on python 3.7 (#971)

* FIX: install on python 3.7

https://github.com/python/cpython/pull/46 /
https://bugs.python.org/issue29463 move the module comment into the
AST node and hence out of the tree which means the 2nd entry in the
tree is now the import rather than the `__version__` string.

Adds nightly on travis.

* BLD: update python tags in setup.py

* CI: switch to 3.7-dev

* CI: allow failure on 3.7 dev

.travis.yml

@@ -8,11 +8,14 @@ python:
   - 3.5
   - 3.6
   - pypy
+  - "3.7-dev"
+
 matrix:
   allow_failures:
     - python: pypy
     - env: TOXENV=cov
     - env: TOXENV=sith
+    - python: 3.7-dev
   include:
     - python: 3.5
       env: TOXENV=cov
@@ -27,4 +30,3 @@ after_script:
       pip install --quiet coveralls;
       coveralls;
       fi
-