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.

Author: mgorny

setup.py

@@ -50,7 +50,7 @@
     "docs": ["furo", "sphinx", "zope.interface", "sphinx-notfound-page"],
     "tests_no_zope": [
         # For regression test to ensure cloudpickle compat doesn't break.
-        "cloudpickle",
+        'cloudpickle; python_implementation == "CPython"',
         # 5.0 introduced toml; parallel was broken until 5.0.2
         "coverage[toml]>=5.0.2",
         "hypothesis",

tests/test_3rd_party.py

@@ -4,13 +4,16 @@
 Tests for compatibility against other Python modules.
 """
 
-import cloudpickle
+import pytest
 
 from hypothesis import given
 
 from .strategies import simple_classes
 
 
+cloudpickle = pytest.importorskip("cloudpickle")
+
+
 class TestCloudpickleCompat(object):
     """
     Tests for compatibility with ``cloudpickle``.