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/atomicwrites/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, buildPythonPackage, fetchPypi }:
+{ stdenv, buildPythonPackage, fetchPypi, pytest }:
 
 buildPythonPackage rec {
   pname = "atomicwrites";
@@ -9,6 +9,10 @@ buildPythonPackage rec {
     sha256 = "75a9445bac02d8d058d5e1fe689654ba5a6556a1dfd8ce6ec55a0ed79866cfa6";
   };
 
+  # Tests depend on pytest but atomicwrites is a dependency of pytest
+  doCheck = false;
+  checkInputs = [ pytest ];
+
   meta = with stdenv.lib; {
     description = "Atomic file writes on POSIX";
     homepage = https://pypi.python.org/pypi/atomicwrites;