Separate output tool error and mypy warnings (stderr, stdout, file)

Today I run mypy > mypy.report to let jenkins parse the output log and present it. However then mypy crashes and prints errors, those also go on stdout and into my report file. It would be nice with some way of separating the output.

example on tool error:

$ mypy --no-incremental --disable-error-code import --follow-imports skip . > mypy.report
 Duplicate module named "benchmark" (also at "a/b/1/benchmark.py")
 a/b/2/benchmark.py: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#mapping-file-paths-to-modules for more info
 a/b/2/benchmark.py: note: Common resolutions include: a) using `--exclude` to avoid checking one of them, b) adding `__init__.py` somewhere, c) using `--explicit-package-bases` or adjusting MYPYPATH

Suggestion:

1. Could these errors be printed to stderr? Should there in that case be an option to do so?
2. option flag to set output_file for the report, but still output errors to stdout.

Inizia tracciando la gestione attuale da parte di mypy, tramite la riga di comando, degli errori degli strumenti e dell'output dei report. Confronta i due comportamenti richiesti —inviare gli errori degli strumenti a stderr oppure configurare un file di report mantenendo stdout— e definisci quale interfaccia debba essere implementata; il lavoro sarà completato quando Jenkins potrà separare in modo affidabile gli errori dal report.