kunit: tool: undo type subscripts for subprocess.Popen

Writing `subprocess.Popen[str]` requires python 3.9+.
kunit.py has an assertion that the python version is 3.7+, so we should
try to stay backwards compatible.

This conflicts a bit with commit 1da2e6220e11 ("kunit: tool: fix
pre-existing `mypy --strict` errors and update run_checks.py"), since
mypy complains like so
> kunit_kernel.py:95: error: Missing type parameters for generic type "Popen" [type-arg]

Note: `mypy --strict --python-version 3.7` does not work.

We could annotate each file with comments like
`# mypy: disable-error-code="type-arg"
but then we might still get nudged to break back-compat in other files.

This patch adds a `mypy.ini` file since it seems like the only way to
disable specific error codes for all our files.

Note: run_checks.py doesn't need to specify `--config_file mypy.ini`,
but I think being explicit is better, particularly since most kernel
devs won't be familiar with how mypy works.

Fixes: 695e26030858 ("kunit: tool: add subscripts for type annotations where appropriate")
Reported-by: SeongJae Park <[e-mail]>
Link: https://lore.kernel.org/linux-kselftest/[e-mail]
Signed-off-by: Daniel Latypov <[e-mail]>
Tested-by: SeongJae Park <[e-mail]>
Reviewed-by: David Gow <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/kunit/kunit_kernel.py

···
 		if stderr:  # likely only due to build warnings
 			print(stderr.decode())
 
-	def start(self, params: List[str], build_dir: str) -> subprocess.Popen[str]:
+	def start(self, params: List[str], build_dir: str) -> subprocess.Popen:
 		raise RuntimeError('not implemented!')
 
 
···
 		kconfig.merge_in_entries(base_kunitconfig)
 		return kconfig
 
-	def start(self, params: List[str], build_dir: str) -> subprocess.Popen[str]:
+	def start(self, params: List[str], build_dir: str) -> subprocess.Popen:
 		kernel_path = os.path.join(build_dir, self._kernel_path)
 		qemu_command = ['qemu-system-' + self._qemu_arch,
 				'-nodefaults',
···
 		kconfig.merge_in_entries(base_kunitconfig)
 		return kconfig
 
-	def start(self, params: List[str], build_dir: str) -> subprocess.Popen[str]:
+	def start(self, params: List[str], build_dir: str) -> subprocess.Popen:
 		"""Runs the Linux UML binary. Must be named 'linux'."""
 		linux_bin = os.path.join(build_dir, 'linux')
 		params.extend(['mem=1G', 'console=tty', 'kunit_shutdown=halt'])

tools/testing/kunit/mypy.ini

···
+[mypy]
+strict = True
+
+# E.g. we can't write subprocess.Popen[str] until Python 3.9+.
+# But kunit.py tries to support Python 3.7+, so let's disable it.
+disable_error_code = type-arg

tools/testing/kunit/run_checks.py

···
 	'kunit_tool_test.py': ['./kunit_tool_test.py'],
 	'kunit smoke test': ['./kunit.py', 'run', '--kunitconfig=lib/kunit', '--build_dir=kunit_run_checks'],
 	'pytype': ['/bin/sh', '-c', 'pytype *.py'],
-	'mypy': ['mypy', '--strict', '--exclude', '_test.py$', '--exclude', 'qemu_configs/', '.'],
+	'mypy': ['mypy', '--config-file', 'mypy.ini', '--exclude', '_test.py$', '--exclude', 'qemu_configs/', '.'],
 }
 
 # The user might not have mypy or pytype installed, skip them if so.