Clarify the need for setup.py with PEP517

There was a misunderstanding that PBRs PEP517 build hooks meant we no
longer needed a setup.py. But it turns out the setuptools build_meta
hooks that we wrap call out to setup.py. This means PBR will continue to
need to be used in conjunction with a setup.py.

This is better for backward compatibiltiy anyway so not a huge loss.

Document this more properly and update the test to reflect it.

Change-Id: If4320da1e89a5c765e0bfe56b724364f9cba34f3

1 files changed, 9 insertions, 2 deletions

diff --git a/doc/source/user/using.rst b/doc/source/user/using.rst
index 7070e01..21aa99b 100644
--- a/doc/source/user/using.rst
+++ b/doc/source/user/using.rst
@@ -37,8 +37,11 @@ something like this::
 ``pyproject.toml``
 ------------------
 
-As an alternative to writing a ``setup.py`` you can instead setup PBR
-through the PEP 517 build-system configuration in ``pyproject.toml``.
+PBR can be configured as a PEP517 build-system in ``pyproject.toml``. This
+currently continues to rely on setuptools which means you need the above
+``setup.py`` file to be present. The main benefits to using a
+``pyproject.toml`` file with PBR are that you can control the versions of
+PBR, setuptools, and wheel that are used avoiding easy_install invocation.
 Your build-system block in ``pyproject.toml`` will need to look something
 like this::
 
@@ -46,6 +49,10 @@ like this::
     requires = ["pbr>=5.7.0", "setuptools>=36.6.0", "wheel"]
     build-backend = "pbr.build"
 
+
+Eventually PBR may grow its own direct support for PEP517 build hooks, but
+until then it will continue to need setuptools and ``setup.py``.
+
 .. _setup_cfg:
 
 ``setup.cfg``