deprecation warnings

Start by running the tests around more/webassets/tests/conftest.py and more/webassets/tests/test_directives.py to reproduce the warning summary. Separate warnings originating in the repository from those under .tox/py37/site-packages, then determine which warnings can be addressed here; done means the in-scope deprecation warnings are no longer emitted.

=============================================== warnings summary ===============================================
more/webassets/tests/conftest.py:7
  /home/jugmac00/Projects/more.webassets/more/webassets/tests/conftest.py:7: PytestDeprecationWarning: @pytest.yield_fixture is deprecated.
  Use @pytest.fixture instead; they are the same.
    @pytest.yield_fixture

more/webassets/tests/test_directives.py::test_webasset_environment
  /home/jugmac00/Projects/more.webassets/.tox/py37/lib/python3.7/site-packages/scss/types.py:6: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3,and in 3.9 it will stop working
    from collections import Iterable

more/webassets/tests/test_directives.py: 154 warnings
  /home/jugmac00/Projects/more.webassets/.tox/py37/lib/python3.7/site-packages/scss/namespace.py:172: DeprecationWarning: inspect.getargspec() is deprecated since Python 3.0, use inspect.signature() or inspect.getfullargspec()
    argspec = inspect.getargspec(function)

more/webassets/tests/test_directives.py::test_webasset_environment
  /home/jugmac00/Projects/more.webassets/.tox/py37/lib/python3.7/site-packages/scss/selector.py:54: FutureWarning: Possible nested set at position 329
    ''', re.VERBOSE | re.MULTILINE)

-- Docs: https://docs.pytest.org/en/stable/warnings.html