Fix Binder build: replace postBuild with requirements.txt

The mybinder.org build failed with "No environment specification
found": repo2docker only treats postBuild as a build step when an
environment specification file (requirements.txt, environment.yml,
pyproject.toml at the repo root, ...) is also present, and this repo
declares dependencies via [tool.poetry], which repo2docker does not
recognize as a specification.

Use .binder/requirements.txt with a single ".[jupyter]" line instead:
it is both the environment specification and the package install, so
the separate postBuild hook is no longer needed. Verified that
`pip install -r .binder/requirements.txt` in a clean venv installs
warpfactory 0.1.0 with ipywidgets.

Author: JohnsterID

.binder/postBuild

@@ -0,0 +1,3 @@
+# repo2docker environment specification: install the package (with the
+# notebook extra) so examples/interactive_explorer.ipynb runs on Binder.
+.[jupyter]