tox: Fix environments to work on both Python versions

Signed-off-by: Vasilis Tsiligiannis <acinonyx@openwrt.gr>

tox.ini

@@ -1,7 +1,13 @@
 [tox]
 envlist = {py2,py3}-flake8,isort,yapf,{py2,py3}-{build,pytest}
 
-[testenv:flake8]
+[testenv:py2-flake8]
+deps =
+    flake8==3.7.7
+skip_install = True
+commands = flake8
+
+[testenv:py3-flake8]
 deps =
     flake8==3.7.7
 skip_install = True
@@ -30,7 +36,12 @@ deps =
 skip_install = True
 commands = yapf -i -r .
 
-[testenv:build]
+[testenv:py2-build]
+skip_install = True
+commands =
+    python setup.py sdist bdist_wheel
+
+[testenv:py3-build]
 skip_install = True
 commands =
     python setup.py sdist bdist_wheel
@@ -43,7 +54,15 @@ deps =
 extras = dev
 commands = pip check
 
-[testenv:pytest]
+[testenv:py2-pytest]
+install_command = python -m pip install --no-deps {opts} {packages}
+deps =
+    -rrequirements-dev.txt
+usedevelop = True
+extras = dev
+commands = pytest
+
+[testenv:py3-pytest]
 install_command = python -m pip install --no-deps {opts} {packages}
 deps =
     -rrequirements-dev.txt