Add support for RxPY 4

First time contributor checklist

• I have read how to contribute to this project
• I have read the code of conduct to this project

Contributor checklist

• I am targeting the master branch and not the release branch (⚠️ very important)
• I have tested my contribution using ./run.sh
• I have tested my contribution using ./build.sh --flatpak-local --flatpak-install
• My contribution is fully baked and ready to be merged as is
• I ensure that all the open issues my contribution fixes are mentioned in the commit message of my first commit using the Fixes #1234 syntax

The flatpak command throws an error unrelated to this PR (happens on release branch as well):

Traceback (most recent call last):
  File "/home/duje/code/gwe/scripts/make_local_manifest.py", line 13, in <module>
    MANIFEST = json.load(open(IN_FILE, encoding='utf-8'))
FileNotFoundError: [Errno 2] No such file or directory: 'flatpak/com.leinardi.gwe.json'
Can't load 'build/flatpak/com.leinardi.gwe.json': Failed to open file “/home/duje/code/gwe/build/flatpak/com.leinardi.gwe.json”: No such file or directory

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Description

This PR intends to add RxPY 4 support. The only important change for gwe appears to be that the module has been renamed from rx to reactivex, and this PR works around this by first trying to import RxPY 4 and falling back to RxPY 3 if the former is not installed.

Type of Changes

	Type
✓	✨ New feature

Related Issue

None