Split buildPythonPackage into setup hooks

This commit splits the `buildPythonPackage` into multiple setup hooks.

Generally, Python packages are built from source to wheels using `setuptools`.
The wheels are then installed with `pip`. Tests were often called with
`python setup.py test` but this is less common nowadays. Most projects
now use a different entry point for running tests, typically `pytest`
or `nosetests`.

Since the wheel format was introduced more tools were built to generate these,
e.g. `flit`. Since PEP 517 is provisionally accepted, defining a build-system
independent format (`pyproject.toml`), `pip` can now use that format to
execute the correct build-system.

In the past I've added support for PEP 517 (`pyproject`) to the Python
builder, resulting in a now rather large builder. Furthermore, it was not possible
to reuse components elsewhere. Therefore, the builder is now split into multiple
setup hooks.

The `setuptoolsCheckHook` is included now by default but in time it should
be removed from `buildPythonPackage` to make it easier to use another hook
(curently one has to pass in `dontUseSetuptoolsCheck`).

pkgs/development/python-modules/py/default.nix

@@ -12,7 +12,11 @@ buildPythonPackage rec {
   # Circular dependency on pytest
   doCheck = false;
 
-  buildInputs = [ setuptools_scm ];
+  nativeBuildInputs = [ setuptools_scm ];
+
+  pythonImportsCheck = [
+    "py"
+  ];
 
   meta = with stdenv.lib; {
     description = "Library with cross-python path, ini-parsing, io, code, log facilities";