Update docs around upgrading dependencies

[sssoleileraaa]

Description

Upgrade our PyQt dev dependency and update readme docs about updating dependencies.

Even though we're no longer seeing #273 occur, we know that the upstream qt bug was fixed in a version of Qt later than the one we've been using.

Resolves #273

[sssoleileraaa]

In order to get around a pip-compile error, I switched from using pip-19.2.2 to pip-18.0

This is the error, in case others run into it:

AttributeError: 'Values' object has no attribute 'process_dependency_links'
Makefile:67: recipe for target 'update-pip-requirements' failed
make: *** [update-pip-requirements] Error 1

[kushaldas]

Debian Buster does not have that version of Qt/PyQt :(

Get:1 https://mirror.0x.sg/debian buster/main amd64 python3-pyqt5 amd64 5.11.3+dfsg-1+b3

[redshiftzero]

hmm good catch, the most recent version buster has is 5.11.3 which does not have the fix https://packages.debian.org/buster/python3-pyqt5 (fix first available in
5.12.4)

[sssoleileraaa]

Okay, so I repurposed the PR to remove the upgrade but keep the docs changes. Ready for review!