Fix PyAV version

[ArtyomVancyan]

Motivation:

In the scope of investigating the #69 issue reported by @mahdiar, it turned out that old versions of the PyAV package are failing with the latest pip and Python versions. Also, the latest PyAV version is compatible with all operating systems supported by this library.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you updated the documentation related to the changes you have made?
• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?