pypi: allow packages in global exclude list if explicitly requested

[dtrodrigues]

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Some python formulae may require packages that are in the global exclude list to function. Allow them to be added if they're explicitly requested.

[BrewTestBot]

Review period will end on 2021-01-19 at 14:07:21 UTC.

[Rylan12]

Looks good to me. Good catch. Thanks, @dtrodrigues!

[BrewTestBot]

Review period ended.