1.1.0-beta.3 - Second Public Beta. Bug fixes and Splash Screen

This is a beta release and may contain bugs or unfinished features. Please
provide bug reports or feedback via: https://github.com/mu-editor/mu/issues/new

• During beta phase, we're moving to a fortnightly release cadence. Expect beta 4 on around the 12th April.
• The final version of the Mu splash screen was delivered. Huge thanks to the extraordinarily talented Steve Hawkes (@hawkz) of The Developer Society for his generous artistic support, patience and humorous approach.
• Thanks to a recent update in PyGame Zero, we're back to using the official package from PyPI, rather than our patched fork, in the installer.
• Both Tim and Carlos have contributed updates, fixes and tests to address a bug affecting Windows users who may have a space in the file path upon which Mu is found. This was a difficult bug to reproduce but Tim did a lot of digging to isolate the cause with as much confidence as is possible when it comes to such things. Carlos did a bunch of thankless and fiddly test related work so testing with spaces in the path is part of our test suite. Work on this is ongoing so expect further improvements in upcoming releases. As always, many thanks for these efforts.
• Tim addressed a wheel/sdist related problem that was causing odd side effects with regard to dependancies.
• A strange bug, where it was not possible to install third-party packages on first run of Mu, opened up a deep rabbit hole of investigation. In the end Tim was able to fix this AND address the source of a warning message from Qt when Mu was starting for the first time.
• The splash screen code was rewritten in such a way that objects relating to the splash screen will always be garbage-collected by Python and destroyed by Qt5. Previously, they existed for the full duration of the application, not really causing any problems, but "in limbo" nonetheless.
• The crash reporting tool has had a minor update so the user is reminded to attach their log file to the bug report, along with an indication of where to find the log file.