[qutebrowser] Support for Qt < 5.12 dropped

[Florian Bruhin]

Heya,

Another item on the path to v2.0.0 done: Support for Qt < 5.12 is now
dropped in qutebrowser. In other words, code for 2.5 years worth of old
Qt versions is now gone. That's *a lot* of code:

  115 files changed, 538 insertions(+), 2322 deletions(-)

For the curious, here's the diff:
https://github.com/qutebrowser/qutebrowser/compare/3bfebc278f010564bb528c3fa078a0fea5fd04ef..1ce5f6ddf0fe5ad837c590795feb2139a5670996

In practice, that means there's much less backwards compatibility and
random workarounds to worry about now, with 5.12 - 5.15 being the
supported Qt span rather than 5.7 - 5.15. Also, less CI jobs, thus less
waiting and less sporadic failures :)

The last time such a cleanup happened is with v1.0.0, more than three
years ago, where support for Qt 5.2 to 5.6 was dropped:
https://github.com/qutebrowser/qutebrowser/commit/852baaa8c30b84b0870b235ff69b2d376ba0613f

Judging from crash reports and such, Qt 5.7 and 5.8 saw very little
usage. Qt 5.9 (LTS) is perhaps a bit more problematic because it's
shipped by Ubuntu 18.04 LTS, but it's unsupported by Qt since May 2020,
the usage is relatively low, and Ubuntu 20.04 LTS has been available for
a while.

The most controversial part perhaps is dropping Qt 5.11, which is used
by the current Debian Stable (Buster). However, consider that:

- It's based on a Chromium version from 2018 with dozens (if not
  hundreds) of security fixes missing, some of which for bugs which are
  known to be exploited in the wild.
- It's not covered by Debian security support:
  https://www.debian.org/releases/buster/amd64/release-notes/ch-information.en.html#browser-security
- It's unsupported upstream by Qt since May 2019.
- It has compatibility issues with various websites (GitHub, Twitch,
  Android Developer documentation, YouTube, ...).

Since no newer Debian Stable is released yet, it's recommended to
install qutebrowser in a virtualenv with a newer version of Qt/PyQt:
https://github.com/qutebrowser/qutebrowser/blob/master/doc/install.asciidoc#installing-qutebrowser-with-virtualenv

Qt 5.12 is still supported by Qt as an LTS until December 2021 and
shipped in e.g. Ubuntu 20.04 LTS - thus, Qt 5.12 will likely stay
supported in qutebrowser until at least mid-2022 or so.

Florian

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