Bundled wheels fix: Flask 2 needs Werkzeug < 3.0.0.

[tmontes]

CONTEXT:

• Running Mu on my dev intel macOS for the purposes of packaging + shipping the next release.
• Development environment created from scratch.

FAILURE:

• Bundled wheel process downloads flask==2.0.3 and non-locked dependencies.
• Brings in Werkzeug 3.0.0.
• import flask fails with it!!!

FIX:

• Tweak wheel downloader to accept "list of pip things to install".
• For flask, specify both flask==2.0.3 and Werkzeug<3.0.0.

[tmontes]

NOTE:

• Tested out on macOS, both from source in "pip install -e ." mode and after pup packaging.
• Flask/web mode working as expected.

PS:

• WRT pygame zero: also working once packaged.
• Does not work from the development venv -- needs minor fix in coming PR.

THUS:

• Will merge regardless of the two Run tests / Test PiOS stretch-2018-03-13 check failures -- everything else looking good.