How to tun a single test with unittest with relative paths?

I have the following project structure:

main.py
mainpackage/
   |---tests__/
         |---test_module1.py
   |---package1/
         |--- module1.py
   |---paackage2/
         |--- module2.py

my test_module1.py looks like this:

import unittest
from ..package1.module1 import toTest
class MyTest(unittest.Testcase):
    def test_mytest(self):
        fu = toTest()
        self.assertEqual(fu,2)

 #more tests omitted

if __name__ == "__main__"
    unittest.main()

But when I run this inside tests

python test_module1.py I obviously get the following error:

ImportError: attempted relative import with no known parent package

But when I move to the folder mainpackage/ and run this:

python -m unittest discover -t .. -v mainpackage.tests.test_module

it runs all tests in that file. However I can not just pick one test of that file. I tried this:

python -m unittest discover -t .. -v mainpackage.tests.test_module.MyTest.test_mytest

Which gave me the following error:

ImportError: Start directory is not importable: