Fix conda solve issue

[jrbourbeau]

Avoid specifying the Python version in multiple places

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[jacobtomlinson]

Thanks for starting this PR @jrbourbeau.

I've pushed a couple more commits to try and unblock the CI:

• Looks like there is a dependency issue in repo2docker builds due to upstream changes in the docker package so I explicitly added the missing deps (Explicit use of chardet, but not an explicit dependency jupyterhub/repo2docker#1065)
• The notebooks were complaining about prophet not being installed so added that to the environment.