Add stdout and stderr to mypy status error

[scriptator]

Relates to #120 : adds further information to the error message so that the mentioned issue can be more easily debugged (e.g. on CI environments)

[dmtucker]

Hey there 👋 Thanks for the contribution!

My only hesitance with this is that it will lead to duplicate output in most cases...

============================ FAILURES ============================
_______________________ demo/test_demo.py ________________________
2: error: Incompatible return value type (got "str", expected "int")
__________________________ test session __________________________
mypy exited with status 1.
stdout: demo/test_demo.py:2: error: Incompatible return value type (got "str", expected "int")
Found 1 error in 1 file (checked 1 source file)

stderr: 
============================== mypy ==============================
Found 1 error in 1 file (checked 1 source file)

I understand #120 to be an issue regarding the use of -x, and in that issue you mention needing this patch for debugging an error on your CI server. Are you using -x in that workflow?

I mainly ask because if the issue was MypyStatusError being raised on a file that wasn't collected by Pytest, that output should already be shown as unmatched_stdout:
https://github.com/dbader/pytest-mypy/blob/3c9d8d5ff8d090e63d80a7a7c6c82d37b0ae2d2a/src/pytest_mypy.py#L329-L331

============================ FAILURES ============================
__________________________ test session __________________________
mypy exited with status 1.
============================== mypy ==============================
demo/uncollected.py:1: error: Incompatible types in assignment (expression has type "int", variable has type "str")
Found 1 error in 1 file (checked 1 source file)