add a ".binder" folder to support mybinder.org and repo2docker builds

[itcarroll]

Description

I don't know for how long #1244 has been a problem, but I don't see how the current repository worked with repo2docker and thus with mybinder.org. The docker builds would find the "environment.yml" file, but would not have the source tree and could not use pip install -e.

This PR adds a .binder folder that includes

1. a "environment.yml" file to constrain the python version. Unfortunately, a minimum version is not recognized by repo2docker, only an explicit version.
2. a "postBuild" to install the package with pip as advised in Python repo is not built if CondaBuildPack is selected jupyterhub/repo2docker#1265

"Ready for review" checklist

• Open PR as draft
• Please review our Pull Request Guide
• Mark "ready for review" after following instructions in the guide

Merge checklist

• PR title is descriptive
• PR body contains links to related and resolved issues (e.g. closes #1)
  - [ ] If needed, CHANGELOG.md updated
  - [ ] If needed, docs and/or README.md updated
  - [ ] If needed, unit tests added
• All checks passing (comment pre-commit.ci autofix if pre-commit is failing)
• At least one approval

[itcarroll]

I confirmed that mybinder.org launch is okay.

[mfisher87]

LGTM, minor suggestion!

[itcarroll]

Note that I tried to switch the root level "environment.yml" to use --group as well, and it does not work.

Installing pip packages: --group dev, --editable .
Usage: __main__.py [options]

ERROR: Invalid requirement: --group dev
__main__.py: error: no such option: --group

That was with pip 26.0.1.

One more review request coming.

[chuckwondo]

Awesome! Thanks @itcarroll!