sorbet: Add an RBI file for PyCall

[issyl0]

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• Part of Use Sorbet typed: strict in all (non-package) files in Homebrew organisation #17297.
• Companion PR to Enable Sorbet typechecking homebrew-formula-analytics#538.
• This is used in Homebrew/formula-analytics to import Python modules, specifically the InfluxDB client we need.
• Sorbet complained that it couldn‘t find the PyCall reference:

/opt/homebrew/Library/Taps/homebrew/homebrew-formula-analytics/lib/pycall-setup.rb:7: Unable to resolve constant PyCall https://srb.help/5002
     7 |  extend PyCall::Import

[Bo98]

PyCall gem is formula-analytics specific: https://github.com/Homebrew/homebrew-formula-analytics/blob/master/Gemfile#L7. Does it work if you move this to a pycall-setup.rbi?