feat: support syntax error messages

[henryiii]

We currently only check on test runs, but if you have a syntax error (in my case, usually a bit of Python 3-only valid syntax that then gets caught in CI on Python 2), it would be nice to have the annotations there as well.

[utgwkk]

It is difficult as a functionality of this plugin, but writing a syntax-check test may be suitable. The below test will fail when source code contains a syntax error.

# For Python 2 compatibility, we use [glob2](https://pypi.org/project/glob2/) library
# instead of built-in glob module.
from glob2 import iglob
import py_compile
import pytest

python_files = iglob('src/**/*.py', recursive=True)

@pytest.mark.parametrize("path", python_files)
def test_syntax(path):
    try:
        py_compile.compile(path, doraise=True)
    except py_compile.PyCompileError as ex:
        pytest.fail(ex.msg)

example result: https://github.com/utgwkk/20210731-sketch-python-problem-matcher/actions/runs/1084361862