More tweaks to enable cross python lockfile

.travis.yml

@@ -42,8 +42,7 @@ install:
   - which $PYTHON
   - $PYTHON --version
   - $PYTHON -m pip install --upgrade pip coveralls pipenv==2018.11.26
-  # Skip lock as we have multiple versions of python
-  - pipenv install --skip-lock --dev
+  - pipenv install --dev
   - $PYTHON setup.py $BDIST
 
 script:

Pipfile

@@ -4,15 +4,18 @@ url = "https://pypi.org/simple"
 verify_ssl = true
 
 [dev-packages]
-black = "==19.10b0"
-mypy = "*"
 pytest-cov = "*"
 pytest-mypy = "*"
 pytest-flake8 = "*"
 pytest-black = "*"
 flake8-isort = "*"
 sphinx-rtd-theme = "*"
 pytest-asyncio = "*"
+# Pinning black stops us having to allow pre-releases globally
+black = "==19.10b0"
+# Without these lines lockfile isn't usable on multiple Python versions and platforms
+mypy = "*"
+atomicwrites = "*"
 
 [scripts]
 # Put coverage here so we don't interfere with debugging in the IDE