Improvements before Qt6

[YakoYakoYokuYoku]

Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. Additionally, make sure you've done all of these things:

• I've followed the contributing guidelines to the best of my understanding
• I've read and understood the contributing guidelines
• I've formatted my code according to Natron's code style
• I've searched the pull requests tracker to ensure that this PR is not a duplicate

PR Description

What type of PR is this? (Check one of the boxes below)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• Breaking change (fix or feature that would cause existing functionality to change)
• My change requires a change to the documentation
  • I have updated the documentation accordingly

What does this pull request do?

This adds a pair of corrections in the build system needed for landing Qt6 support, they affect mostly the Python bindings generation and are backwards compatible with Qt5.

Show a few screenshots (if this is a visual change)

N/A.

Have you tested your changes (if applicable)? If so, how?

By building and running Natron.

Futher details of this pull request

This is a stopgap before #1019 can be merged.

[YakoYakoYokuYoku]

Will merge tomorrow if no objections are raised.

[acolwell]

In general I'm supportive of these changes and just have a few comments

[devernay]

@YakoYakoYokuYoku please avoid force-pushing without an approval from the reviewer, except for minor stuff (e.g. typo, or 1-line trivial fix), even if you addressed the reviewer's comments. There's no hurry in merging in general.