tox: Add PYTHON to passenv.

tox.ini

@@ -7,6 +7,7 @@
 envlist = unittests,misc,pep257,pyflakes,pep8,mccabe,pylint,pyroma,check-manifest
 
 [testenv]
+passenv = PYTHON
 basepython = python3
 
 [testenv:mkvenv]
@@ -17,7 +18,7 @@ usedevelop = true
 [testenv:unittests]
 # https://bitbucket.org/hpk42/tox/issue/246/ - only needed for Windows though
 setenv = QT_QPA_PLATFORM_PLUGIN_PATH={envdir}/Lib/site-packages/PyQt5/plugins/platforms
-passenv = DISPLAY XAUTHORITY HOME
+passenv = PYTHON DISPLAY XAUTHORITY HOME
 deps =
     -r{toxinidir}/requirements.txt
     py==1.4.28
@@ -31,7 +32,7 @@ commands =
     {envpython} -m py.test --strict -rfEsw {posargs}
 
 [testenv:coverage]
-passenv = DISPLAY XAUTHORITY HOME
+passenv = PYTHON DISPLAY XAUTHORITY HOME
 deps =
     {[testenv:unittests]deps}
     coverage==3.7.1
@@ -65,7 +66,7 @@ commands =
 [testenv:pep257]
 skip_install = true
 deps = pep257==0.5.0
-passenv = LANG
+passenv = PYTHON LANG
 # Disabled checks:
 # D102: Docstring missing, will be handled by others
 # D209: Blank line before closing """ (removed from PEP257)
@@ -138,7 +139,7 @@ commands =
 [testenv:smoke]
 # https://bitbucket.org/hpk42/tox/issue/246/ - only needed for Windows though
 setenv = QT_QPA_PLATFORM_PLUGIN_PATH={envdir}/Lib/site-packages/PyQt5/plugins/platforms
-passenv = DISPLAY XAUTHORITY HOME USERNAME USER
+passenv = PYTHON DISPLAY XAUTHORITY HOME USERNAME USER
 deps =
     -r{toxinidir}/requirements.txt
 commands =