fix: Addded flask-cors for web plugin to use

[matlads]

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• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

Add flask-cors python package to the image.

Benefits of this PR and context:

This PR adds to the python packages available to the app. The web plugin uses flask-cors

How Has This Been Tested?

1. Bulit a new docker image
2. Run it and changed the config and added cors support to the web plugin.
3. Used an external beet web frontend (beetle) to play a track.
4. No changes to any code

Source / References:

web plugin documentation

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https://ci-tests.linuxserver.io/lspipepr/beets/2.2.0-pkg-a8684ada-dev-78bee30ac66edadc8f8db8228830d10bb9566666-pr-135/shellcheck-result.xml

Tag	Passed
amd64-2.2.0-pkg-a8684ada-dev-78bee30ac66edadc8f8db8228830d10bb9566666-pr-135	✅
arm64v8-2.2.0-pkg-a8684ada-dev-78bee30ac66edadc8f8db8228830d10bb9566666-pr-135	✅

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https://ci-tests.linuxserver.io/lspipepr/beets/2.2.0-pkg-803f2587-dev-ee7b22b1820e4109e2d7d4dccddf650c29f94f2f-pr-135/shellcheck-result.xml

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arm64v8-2.2.0-pkg-803f2587-dev-ee7b22b1820e4109e2d7d4dccddf650c29f94f2f-pr-135	✅

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https://ci-tests.linuxserver.io/lspipepr/beets/2.3.0-pkg-4d511b11-dev-8be3df58c8477f608dd6017f7d0c6eb3593e6568-pr-135/shellcheck-result.xml

Tag	Passed
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arm64v8-2.3.0-pkg-4d511b11-dev-8be3df58c8477f608dd6017f7d0c6eb3593e6568-pr-135	✅

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https://ci-tests.linuxserver.io/lspipepr/beets/2.3.1-pkg-6632ad8f-dev-4891162df5f98abb6406f1490ff856ba18195c88-pr-135/shellcheck-result.xml

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arm64v8-2.3.1-pkg-6632ad8f-dev-4891162df5f98abb6406f1490ff856ba18195c88-pr-135	✅

[LinuxServer-CI]

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https://ci-tests.linuxserver.io/lspipepr/beets/2.3.1-pkg-8f002f01-dev-a393e8a1d214ddded8f3e13a31b46d54c7d1fa24-pr-135/shellcheck-result.xml

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amd64-2.3.1-pkg-8f002f01-dev-a393e8a1d214ddded8f3e13a31b46d54c7d1fa24-pr-135	✅
arm64v8-2.3.1-pkg-8f002f01-dev-a393e8a1d214ddded8f3e13a31b46d54c7d1fa24-pr-135	✅