Port tests to pytest

Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.

This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.

setup.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 
-# Setup script for PyPI; use CMakeFile.txt to build the example application
+# Setup script for PyPI; use CMakeFile.txt to build extension modules
 
 from setuptools import setup
 from pybind11 import __version__