Pin to older coverage version until the bug is fixed

See nedbat/coveragepy#909

.travis.yml

@@ -9,7 +9,7 @@ python:
   - pypy
   - pypy3
 install:
-  - pip install coverage coveralls coverage-python-version
+  - pip install coverage coveralls coverage-python-version -cconstraints.txt
   - pip install freezegun mock flake8
   - pip install --no-deps -e .
 script:

constraints.txt

@@ -0,0 +1,2 @@
+# workaround for https://github.com/nedbat/coveragepy/issues/909
+coverage < 5.0

tox.ini

@@ -20,6 +20,7 @@ deps =
     mock
     coverage
     coverage-python-version
+    -cconstraints.txt
 skip_install = true
 commands =
     pip install --no-deps -e .
@@ -31,6 +32,7 @@ deps =
     freezegun
     coverage
     coverage-python-version
+    -cconstraints.txt
 skip_install = true
 commands =
     {[testenv:coverage]commands}