Update tox.ini to support Windows, and adjust install instructions

INSTALL.asciidoc

@@ -331,6 +331,12 @@ https://docs.python.org/3/library/venv.html[virtual environment]:
 $ tox -e mkvenv
 ----
 
+On Windows, run tox with the 'mkvenv-win' option, however make sure that ONLY Python3 is in your PATH before running tox.
+
+----
+$ tox -e mkvenv-win
+----
+
 This installs all needed Python dependencies in a `.venv` subfolder. The
 system-wide Qt5/PyQt5 installations are symlinked into the virtual environment.
 

tox.ini

@@ -50,6 +50,15 @@ usedevelop = true
 deps =
     -r{toxinidir}/requirements.txt
 
+# This is used for Windows, since binary name is different
+[testenv:mkvenv-win]
+basepython = python.exe
+commands = {envpython} scripts/link_pyqt.py --tox {envdir}
+envdir = {toxinidir}/.venv
+usedevelop = true
+deps =
+    -r{toxinidir}/requirements.txt
+
 # This is undocumented, but it's a common typo, so let's make it work
 [testenv:mkenv]
 basepython = {[testenv:mkvenv]basepython}