WIP: PyPy support (#527)

This commit includes modifications that are needed to get pybind11 to work with PyPy. The full test suite compiles and runs except for a last few functions that are commented out (due to problems in PyPy that were reported on the PyPy bugtracker).

Two somewhat intrusive changes were needed to make it possible: two new tags ``py::buffer_protocol()`` and ``py::metaclass()`` must now be specified to the ``class_`` constructor if the class uses the buffer protocol and/or requires a metaclass (e.g. for static properties).

Note that this is only for the PyPy version based on Python 2.7 for now. When the PyPy 3.x has caught up in terms of cpyext compliance, a PyPy 3.x patch will follow.

tests/conftest.py

@@ -10,6 +10,8 @@ import difflib
 import re
 import sys
 import contextlib
+import platform
+import gc
 
 _unicode_marker = re.compile(r'u(\'[^\']*\')')
 _long_marker = re.compile(r'([0-9])L')
@@ -176,6 +178,13 @@ def suppress(exception):
         pass
 
 
+def gc_collect():
+    ''' Run the garbage collector twice (needed when running
+    reference counting tests with PyPy) '''
+    gc.collect()
+    gc.collect()
+
+
 def pytest_namespace():
     """Add import suppression and test requirements to `pytest` namespace"""
     try:
@@ -190,6 +199,7 @@ def pytest_namespace():
         from pybind11_tests import have_eigen
     except ImportError:
         have_eigen = False
+    pypy = platform.python_implementation() == "PyPy"
 
     skipif = pytest.mark.skipif
     return {
@@ -200,6 +210,8 @@ def pytest_namespace():
                                            reason="eigen and/or numpy are not installed"),
         'requires_eigen_and_scipy': skipif(not have_eigen or not scipy,
                                            reason="eigen and/or scipy are not installed"),
+        'unsupported_on_pypy': skipif(pypy, reason="unsupported on PyPy"),
+        'gc_collect': gc_collect
     }