tests do not work

[bertsky]

___________________________________________________ ERROR collecting tests/test_crop.py ____________________________________________________
ImportError while importing test module '/build/ocrd_anybaseocr/tests/test_crop.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_crop.py:10: in <module>
    from tests.base import TestCase, assets, main, copy_of_directory
E   ModuleNotFoundError: No module named 'tests.base'
__________________________________________________ ERROR collecting tests/test_dewarp.py ___________________________________________________
ImportError while importing test module '/build/ocrd_anybaseocr/tests/test_dewarp.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_dewarp.py:12: in <module>
    from tests.base import TestCase, assets, main, copy_of_directory
E   ModuleNotFoundError: No module named 'tests.base'
___________________________________________________ ERROR collecting tests/test_smoke.py ___________________________________________________
ImportError while importing test module '/build/ocrd_anybaseocr/tests/test_smoke.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_smoke.py:3: in <module>
    from tests.base import main, CapturingTestCase as TestCase
E   ModuleNotFoundError: No module named 'tests.base'

[kba]

fixed by #89