pybuild mysteriously does not copy all files

[Ole Streicher <olebole@debian.org>]

Hi,

I am not sure whether this is a bug in dh-python or if something is
wrong with my package (python-asdf). The package has an (almost)
standard setup, so dh_build is not overridden.

However, when I try to package, some subdirectories are not copied;
namely `asdf/external`:

----------------------------8<----------------------------------
   dh_auto_build -O--buildsystem=pybuild
I: pybuild base:217: /usr/bin/python3 setup.py build 
running build
running build_py
creating /build/python-asdf-2.4.2/.pybuild/cpython3_3.7_asdf/build/asdf
[...]
----------------------------8<----------------------------------

The subdir `asdf/external` which is in the source is not created and not
filled during this process.

However, when I run `python3 setup.py build` or `pybuild --build`
manually, everything is correct:

----------------------------8<----------------------------------
$ python3 setup.py build -vvv
running build
running build_py
creating build
creating build/lib
creating build/lib/asdf
copying asdf/yamlutil.py -> build/lib/asdf
[...]
creating build/lib/asdf/extern
copying asdf/extern/RangeHTTPServer.py -> build/lib/asdf/extern
copying asdf/extern/__init__.py -> build/lib/asdf/extern
copying asdf/extern/atomicfile.py -> build/lib/asdf/extern
copying asdf/extern/decorators.py -> build/lib/asdf/extern
[...]
----------------------------8<----------------------------------

Is there any idea what happens here? Why does pybuild (or setup.py)
behave differently when called from dh_python than directly?

How can I ensure that all files are copied during dh_auto_build?

Best regards

Ole