Read $PYTHON in every tox.ini environment

See #2341

doc/install.asciidoc

@@ -379,8 +379,8 @@ local Qt install instead of installing PyQt in the virtualenv. However, unless
 you have a new QtWebKit or QtWebEngine available, qutebrowser will not work. It
 also typically means you'll be using an older release of QtWebEngine.
 
-On Windows, run `tox -e 'mkvenv-win' instead, however make sure that ONLY
-Python3 is in your PATH before running tox.
+On Windows, run `set PYTHON=C:\path\to\python.exe` (CMD) or ``$Env:PYTHON =
+"..."` (Powershell) first.
 
 Creating a wrapper script
 ~~~~~~~~~~~~~~~~~~~~~~~~~

tox.ini

@@ -35,16 +35,7 @@ commands =
 # other envs
 
 [testenv:mkvenv]
-basepython = python3
-commands = {envpython} scripts/link_pyqt.py --tox {envdir}
-envdir = {toxinidir}/.venv
-usedevelop = true
-deps =
-    -r{toxinidir}/requirements.txt
-
-# This is used for Windows, since binary name is different
-[testenv:mkvenv-win]
-basepython = python.exe
+basepython = {env:PYTHON:python3}
 commands = {envpython} scripts/link_pyqt.py --tox {envdir}
 envdir = {toxinidir}/.venv
 usedevelop = true
@@ -61,7 +52,7 @@ deps = {[testenv:mkvenv]deps}
 
 # Virtualenv with PyQt5 from PyPI
 [testenv:mkvenv-pypi]
-basepython = python3
+basepython = {env:PYTHON:python3}
 envdir = {toxinidir}/.venv
 commands = {envpython} -c ""
 usedevelop = true
@@ -69,19 +60,9 @@ deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/misc/requirements/requirements-pyqt.txt
 
-# This is used for Windows, since binary name is different
-[testenv:mkvenv-win-pypi]
-basepython = python.exe
-commands = {envpython} -c ""
-envdir = {toxinidir}/.venv
-usedevelop = true
-deps =
-    -r{toxinidir}/requirements.txt
-    -r{toxinidir}/misc/requirements/requirements-pyqt.txt
-
 [testenv:misc]
 ignore_errors = true
-basepython = python3
+basepython = {env:PYTHON:python3}
 # For global .gitignore files
 passenv = HOME
 deps =
@@ -91,7 +72,7 @@ commands =
     {envpython} scripts/dev/misc_checks.py spelling
 
 [testenv:vulture]
-basepython = python3
+basepython = {env:PYTHON:python3}
 deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/misc/requirements/requirements-vulture.txt
@@ -101,7 +82,7 @@ commands =
     {envpython} scripts/dev/run_vulture.py
 
 [testenv:vulture-pyqtlink]
-basepython = python3
+basepython = {env:PYTHON:python3}
 deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/misc/requirements/requirements-vulture.txt
@@ -137,7 +118,7 @@ commands =
     {envpython} scripts/dev/run_pylint_on_tests.py {toxinidir} --output-format=colorized --reports=no {posargs}
 
 [testenv:pylint-master]
-basepython = python3
+basepython = {env:PYTHON:python3}
 passenv = {[testenv:pylint]passenv}
 deps =
     -r{toxinidir}/requirements.txt
@@ -149,7 +130,7 @@ commands =
     {envpython} scripts/dev/run_pylint_on_tests.py --output-format=colorized --reports=no {posargs}
 
 [testenv:flake8]
-basepython = python3
+basepython = {env:PYTHON:python3}
 passenv =
 deps =
     -r{toxinidir}/requirements.txt
@@ -158,7 +139,7 @@ commands =
     {envpython} -m flake8 {posargs:qutebrowser tests scripts}
 
 [testenv:pyroma]
-basepython = python3
+basepython = {env:PYTHON:python3}
 passenv =
 deps =
     -r{toxinidir}/misc/requirements/requirements-pyroma.txt
@@ -166,7 +147,7 @@ commands =
     {envdir}/bin/pyroma .
 
 [testenv:check-manifest]
-basepython = python3
+basepython = {env:PYTHON:python3}
 passenv =
 deps =
      -r{toxinidir}/misc/requirements/requirements-check-manifest.txt
@@ -174,7 +155,7 @@ commands =
     {envdir}/bin/check-manifest --ignore 'qutebrowser/git-commit-id,qutebrowser/html/doc,qutebrowser/html/doc/*,*/__pycache__'
 
 [testenv:docs]
-basepython = python3
+basepython = {env:PYTHON:python3}
 whitelist_externals = git
 passenv = TRAVIS TRAVIS_PULL_REQUEST
 deps =