* Do not require cloudpickle for PyPy
The cloudpickle package relies on CPython implementation details,
and does not even import on PyPy:
```
ImportError while importing test module '/tmp/attrs/tests/test_3rd_party.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/pypy3.8/importlib/__init__.py:127: in import_module
return _bootstrap._gcd_import(name[level:], package, level)
tests/test_3rd_party.py:7: in <module>
import cloudpickle
.tox/pypy3/lib/pypy3.8/site-packages/cloudpickle/__init__.py:4: in <module>
from cloudpickle.cloudpickle import * # noqa
.tox/pypy3/lib/pypy3.8/site-packages/cloudpickle/cloudpickle.py:57: in <module>
from .compat import pickle
.tox/pypy3/lib/pypy3.8/site-packages/cloudpickle/compat.py:13: in <module>
from _pickle import Pickler # noqa: F401
E ModuleNotFoundError: No module named '_pickle'
```
Disable the dependency for PyPy and make the test handle missing
cloudpickle gracefully.
* Enable testing on pypy-3.8
* add a news entry
Co-authored-by: Hynek Schlawack <hs@ox.cx>
Michał Górny