[mixer:vod] relax _VALID_URL (fixes #21531)

[mikf]

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Changed the _VALID_URL regex pattern to match new VOD URLs as seen in #21531
e.g. https://mixer.com/Midgalicis?vod=IxFno1rqC0S_XJ1a2yGgNw

[987computer]

I recompiled it with the updated py file and it seems to have worked well. Hope this fix comes with the next officially released version.