Unit test generators are unsupported

[njsaunders]

Due to "limited support" in xmlrunner for unittest.TestCase.subTest and the fact that pybuilder uses xmlrunner to run unit tests, unit test generators are unsupported in pybuilder, which is a shame because they're a really nice feature :)

Expected Behavior

Tests run - 3 pass, 3 fail.

Actual Behavior

BUILD FAILED - TypeError: issubclass() arg 2 must be a class or tuple of classes (xmlrunner/result.py:375)

PyBuilder version 0.12.6
Build started at 2020-07-17 12:58:14
------------------------------------------------------------
[INFO]  Building testcase_pyb version 1.0.dev0
[INFO]  Executing build in /Users/neil/testcase_pyb
[INFO]  Going to execute task publish
[INFO]  Processing plugin packages 'coverage~=5.0' to be installed with {'upgrade': True}
[INFO]  Processing plugin packages 'flake8~=3.7' to be installed with {'upgrade': True}
[INFO]  Processing plugin packages 'pypandoc~=1.4' to be installed with {'upgrade': True}
[INFO]  Processing plugin packages 'setuptools>=38.6.0' to be installed with {'upgrade': True}
[INFO]  Processing plugin packages 'twine>=1.15.0' to be installed with {'upgrade': True}
[INFO]  Processing plugin packages 'unittest-xml-reporting~=2.5.2' to be installed with {'upgrade': True}
[INFO]  Processing plugin packages 'wheel>=0.34.0' to be installed with {'upgrade': True}
[INFO]  Creating target 'build' VEnv in '/Users/neil/testcase_pyb/target/venv/build/cpython-3.7.7.final.0'
[INFO]  Creating target 'test' VEnv in '/Users/neil/testcase_pyb/target/venv/test/cpython-3.7.7.final.0'
[INFO]  Requested coverage for tasks: pybuilder.plugins.python.unittest_plugin:run_unit_tests
[INFO]  Running unit tests
[INFO]  Executing unit tests from Python modules in /Users/neil/testcase_pyb/src/unittest/python
------------------------------------------------------------
BUILD FAILED - TypeError: issubclass() arg 2 must be a class or tuple of classes (xmlrunner/result.py:375)
------------------------------------------------------------
Build finished at 2020-07-17 12:58:59
Build took 44 seconds (44762 ms)```

Steps to Reproduce the Problem

1. Initialise a new pybuilder project
2. Write the following to ./src/unittest/python/generator_tests.py (The example code taken from here)

import unittest

class NumbersTest(unittest.TestCase):

    def test_even(self):
        """
        Test that numbers between 0 and 5 are all even.
        """
        for i in range(0, 6):
            with self.subTest(i=i):
                self.assertEqual(i % 2, 0)

3. Run the tests with pyb -v

Specifications

• Version: pyb 0.12.6, Python 3.7.7
• Platform: MacOS Catalina (10.15.5)

[arcivanov]

Firstly: https://github.com/xmlrunner/unittest-xml-reporting#limited-support-for-unittesttestcasesubtest