replace test tags with pytest.mark

replace test tags with pytest.mark

replaced the __tags__ class attribute and the
--exclude-tags / --include-tags test runner options
with regular pytest.mark names
so that we can take advantage of mark expressions.
options --nomemory, --notimingintensive, --backend-only,
--exclude-tags, --include-tags remain as legacy but
make use of pytest mark for implemementation.

Added a "mypy" mark for the section of tests that are doing mypy
integration tests.

The __backend__ and __sparse_backend__ class attributes also
use pytest marks for their implementation, which also allows
the marks "backend" and "sparse_backend" to be used explicitly.

Also removed the no longer used "--cdecimal" option as this was
python 2 specific.

in theory, the usage of pytest marks could expand such that
the whole exclusions system would be based on it, but this
does not seem to have any advantage at the moment.

Change-Id: Ideeb57d9d49f0efc7fc0b6b923b31207ab783025