Go back to Qt 5.12 for tox/pyinstaller

This reverts commit 548b5a17d7.
This reverts commit fe32e00a4e.

Due to QTBUG-77364 (broken keyboard focus), we can't upgrade to qtbase 5.13.1.

However, with 5.13.0 (and QtWebEngine 5.13.1), we run into two more issues:

- Segfaults in QQuickWindowPrivate::updateDirtyNode() when logging into a
  Google account (especially on macOS).
- Qt 5.13.0 with the old PyQt5 exit scheme segfaults on exit on Windows.
  However, pyqt5_enable_new_onexit_scheme() was added in PyQt 5.13.1.

See https://bugreports.qt.io/browse/QTBUG-77364
See https://www.riverbankcomputing.com/news/pyqt-5131
Reopens #4918

README.asciidoc

@@ -97,8 +97,9 @@ Requirements
 The following software and libraries are required to run qutebrowser:
 
 * https://www.python.org/[Python] 3.5.2 or newer (3.6 recommended)
-* https://www.qt.io/[Qt] 5.7.1 or newer (5.12 recommended due to https://bugreports.qt.io/browse/QTBUG-76913[segfaults on media pages with Qt 5.13]); support for < 5.9
-  will be dropped soon) with the following modules:
+* https://www.qt.io/[Qt] 5.7.1 or newer (5.12 recommended due to severe bugs in
+  Qt 5.13.0 and .1); support for < 5.9 will be dropped soon) with the following
+  modules:
   - QtCore / qtbase
   - QtQuick (part of qtbase in some distributions)
   - QtSQL (part of qtbase in some distributions)

doc/changelog.asciidoc

@@ -15,15 +15,6 @@ breaking changes (such as renamed commands) can happen in minor releases.
 // `Fixed` for any bug fixes.
 // `Security` to invite users to upgrade in case of vulnerabilities.
 
-v1.8.1 (unreleased)
--------------------
-
-Changed
-~~~~~~~
-
-- Builds using `tox -e mkvenv-pypi` now also use Qt 5.13.
-
-
 v1.8.0 (2019-09-25)
 -------------------
 

tox.ini

@@ -63,7 +63,7 @@ commands = {envpython} -c ""
 usedevelop = true
 deps =
     -r{toxinidir}/requirements.txt
-    -r{toxinidir}/misc/requirements/requirements-pyqt.txt
+    -r{toxinidir}/misc/requirements/requirements-pyqt-5.12.txt
 
 [testenv:misc]
 ignore_errors = true
@@ -178,7 +178,7 @@ passenv = APPDATA HOME
 deps =
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/misc/requirements/requirements-pyinstaller.txt
-    -r{toxinidir}/misc/requirements/requirements-pyqt.txt
+    -r{toxinidir}/misc/requirements/requirements-pyqt-5.12.txt
 commands =
     {envbindir}/pyinstaller --noconfirm misc/qutebrowser.spec