Fix pytestmark syntax in reference.rst
pytest 4.3.1 throws an error if `pytestmark` is set to a tuple of marks; it appears to insist on a list.
With `pytestmark = [pytest.mark.api, pytest.mark.good]`:
```
============================== test session starts ==============================
platform win32 -- Python 3.6.3, pytest-4.3.1, py-1.8.0, pluggy-0.9.0
rootdir: C:\Temp\git\sphobjinv, inifile: tox.ini
plugins: timeout-1.3.3
collected 48 items / 41 deselected / 7 selected
tests\test_api_good.py ....... [100%]
==================== 7 passed, 41 deselected in 0.15 seconds ====================
```
With `pytestmark = (pytest.mark.api, pytest.mark.good)`:
```
==================================== ERRORS =====================================
____________________ ERROR collecting tests/test_api_good.py ____________________
env\lib\site-packages\_pytest\runner.py:226: in from_call
result = func()
env\lib\site-packages\_pytest\runner.py:289: in <lambda>
call = CallInfo.from_call(lambda: list(collector.collect()), "collect")
env\lib\site-packages\_pytest\python.py:435: in collect
self._inject_setup_module_fixture()
env\lib\site-packages\_pytest\python.py:447: in _inject_setup_module_fixture
setup_module = _get_non_fixture_func(self.obj, "setUpModule")
env\lib\site-packages\_pytest\python.py:255: in obj
self.own_markers.extend(get_unpacked_marks(self.obj))
env\lib\site-packages\_pytest\mark\structures.py:244: in get_unpacked_marks
return normalize_mark_list(mark_list)
env\lib\site-packages\_pytest\mark\structures.py:259: in normalize_mark_list
raise TypeError("got {!r} instead of Mark".format(mark))
E TypeError: got (MarkDecorator(mark=Mark(name='api', args=(), kwargs={})), MarkDecorator(mark=Mark(name='good', args=(), kwargs={}))) instead of Mark
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!
==================== 19 deselected, 1 error in 0.27 seconds =====================
```
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