Call mypy with newly added parameter flush_errors

Unfortunately the target function does not have any docstring in mypy,
but it is typed as Optional[] which means the function should accept
None as a value. Doing so seems to suppress issues reported in #6.

Quick skim of the culprit PR in mypy
(https://github.com/python/mypy/pull/4396/files) shows the code really
counts with None being passed and adjusting appropriately, citing:

> If we were not given a flush_errors, we use one that will populate
> those fields for callers that want the traditional API.

pytest_mypy.py

@@ -55,7 +55,7 @@ def runtest(self):
         sources, options = mypy.main.process_options(mypy_argv)
 
         try:
-            res = mypy.main.type_check_only(sources, None, options)
+            res = mypy.main.type_check_only(sources, None, options, flush_errors=None)
             errors = res.errors
         except mypy.errors.CompileError as e:
             errors = e.messages

setup.py

@@ -23,7 +23,7 @@ def read(fname):
     description='Mypy static type checker plugin for Pytest',
     long_description=read('README.rst'),
     py_modules=['pytest_mypy'],
-    install_requires=['pytest>=2.9.2', 'mypy>=0.470'],
+    install_requires=['pytest>=2.9.2', 'mypy>=0.570'],
     classifiers=[
         'Development Status :: 4 - Beta',
         'Framework :: Pytest',