Python: Support streamed compression with the Compressor object (#448) This adds `flush` and `finish` methods to the `Compressor` object in the extension module, renames the `compress` method to `process`, and updates that method to only process data. Now, one or more `process` calls followed by a `finish` call will be equivalent to a module-level `compress` call. Note: To maximize the compression efficiency (and match underlying Brotli behavior, the `Compressor` object `process` method does not guarantee all input is immediately written to output. To ensure immediate output, call `flush` to manually flush the compression buffer. Extraneous flushing can increase the size, but may be required when processing streaming data. Progress on #191