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.

.gitignore

@@ -3,10 +3,9 @@ CMakeFiles
 Makefile
 cmake_install.cmake
 .DS_Store
-/example/example*.so
-/example/example.cpython*.so
-/example/example.pyd
-/example/example*.dll
+*.so
+*.pyd
+*.dll
 *.sln
 *.sdf
 *.opensdf
@@ -32,3 +31,4 @@ MANIFEST
 /dist
 /build
 /cmake/
+.cache/