sorbet: Add an RBI file for PyCall

- 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, so let’s teach Sorbet about
  it.

Library/Homebrew/sorbet/rbi/pycall-setup.rbi

@@ -0,0 +1,12 @@
+# typed: true
+
+# This is required for the `formula-analytics` tap's usage of `PyCall`.
+module PyCall
+  module Import
+    def self.pyfrom(*args); end
+
+    def self.import(*args); end
+  end
+
+  class PyError; end
+end