Travis: only need to avoid tox==3.7.0

tox 3.7.1 (or whatever's next) will fix the issue described in 976bca1.

.travis.yml

@@ -60,8 +60,8 @@ matrix:
 cache: pip
 
 install:
-  # pin tox to avoid https://github.com/tox-dev/tox/issues/1160
-  - pip install tox~=3.6.1
+  # avoid https://github.com/tox-dev/tox/issues/1160
+  - pip install tox!=3.7.0
 
 script:
   - tox

Pipfile

@@ -19,7 +19,7 @@ flake8 = "*"
 mock = "*"
 sphinx = "*"
 sphinx-rtd-theme = "*"
-tox = "~=3.6.1"
+tox = "!=3.7.0"
 twine = "*"
 
 [requires]