Add a mkvenv-pypi-old environment

Fixes #4038
See #3662

doc/install.asciidoc

@@ -392,6 +392,10 @@ https://docs.python.org/3/library/venv.html[virtual environment]:
 $ tox -e mkvenv-pypi
 ----
 
+If your system comes with Python 3.5.3 or older (such as Ubuntu 16.04 LTS), use
+`tox -e mkvenv-pypi-old` instead. This installs an older Qt version (5.10) due
+to bugs in newer versions.
+
 This installs all needed Python dependencies in a `.venv` subfolder.
 
 This comes with an up-to-date Qt/PyQt including QtWebEngine, but has a few

tox.ini

@@ -62,6 +62,19 @@ deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/misc/requirements/requirements-pyqt.txt
 
+# Older PyQt for Python 3.5
+# 5.11.2: https://www.riverbankcomputing.com/pipermail/pyqt/2018-July/040511.html
+# 5.10.1: https://github.com/qutebrowser/qutebrowser/issues/3662
+[testenv:mkvenv-pypi-old]
+basepython = {env:PYTHON:python3.5}
+envdir = {toxinidir}/.venv
+commands = {envpython} -c ""
+usedevelop = true
+deps =
+    -r{toxinidir}/requirements.txt
+    PyQt5==5.10
+    sip==4.19.8
+
 [testenv:misc]
 ignore_errors = true
 basepython = {env:PYTHON:python3}