Pin python-mpv to 0.5.2

Python-mpv updated with code that breaks the Time Machine. So it's now pinned to a previous, working release.

What's Changed

• update the release number to v1.2.3.post2 by @eichblatt in #266
• issue #269 resolved ☑️ by @Sidahmed-ben in #271
• Closes #280 by @eichblatt in #292
• updating to 1.2.3.post3 by @eichblatt in #293

New Contributors

• @Sidahmed-ben made their first contribution in #271

Full Changelog: v1.2.3.post2...v1.2.3.post3