Replace requirements.txt with requirements-dev.txt and use pip-tools.

requirements-dev.in

@@ -0,0 +1,6 @@
+django
+six
+tox
+pytest
+flake8
+isort

requirements-dev.txt

@@ -1,6 +1,30 @@
--r requirements.txt
-
-tox
-pytest
-flake8
-isort
+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile requirements-dev.in
+#
+attrs==19.3.0             # via pytest
+django==2.2.7
+entrypoints==0.3          # via flake8
+filelock==3.0.12          # via tox
+flake8==3.7.9
+importlib-metadata==0.23  # via pluggy, pytest, tox
+isort==4.3.21
+mccabe==0.6.1             # via flake8
+more-itertools==7.2.0     # via pytest, zipp
+packaging==19.2           # via pytest, tox
+pluggy==0.13.1            # via pytest, tox
+py==1.8.0                 # via pytest, tox
+pycodestyle==2.5.0        # via flake8
+pyflakes==2.1.1           # via flake8
+pyparsing==2.4.5          # via packaging
+pytest==5.3.0
+pytz==2019.3              # via django
+six==1.13.0
+sqlparse==0.3.0           # via django
+toml==0.10.0              # via tox
+tox==3.14.1
+virtualenv==16.7.8        # via tox
+wcwidth==0.1.7            # via pytest
+zipp==0.6.0               # via importlib-metadata

requirements.txt

@@ -1,2 +0,0 @@
-django
-six