Add an option to use mamba

[prusse-martin]

Related issue: #133

@nicoddemus @marcelotrevisani
Are you guys with this approach?

W:\my\Projects\mamba-test (master -> origin)
λ mamba devenv
!!! __call_conda_env_update proc_exe: conda-devenv
!!! __call_conda_env_update proc_exe: python
!!! __call_conda_env_update proc_exe: mamba
> Executing: mamba env update --file W:\my\Projects\mamba-test\environment.yml --prune
!!! __call_conda_env_update command: ['mamba', 'env', 'update', '--file', 'W:\\my\\Projects\\mamba-test\\environment.yml', '--prune']
!!! _call_conda env_manager: mamba
...
Transaction

  Prefix: W:\my\envs\mamba-devenv

  All requested packages already installed



Looking for: ["python[version='>=3.6']"]


Preparing transaction: done
Verifying transaction: done
Executing transaction: done
#
# To activate this environment, use
#
#     $ conda activate mamba-devenv
#
# To deactivate an active environment, use
#
#     $ conda deactivate
...

[nicoddemus]

Nice!

Besides defaulting to conda as the env manager, we also need to update the docs.

[prusse-martin]

The relevant rendered docs:

https://conda-devenv--134.org.readthedocs.build/en/134/readme.html#mamba-support
https://conda-devenv--134.org.readthedocs.build/en/134/usage.html#options
https://conda-devenv--134.org.readthedocs.build/en/134/usage.html#env-manager

[prusse-martin]

#136 (comment)

After merging this I pretend to rebase #134 to drop python 3.6 there.

(For some reason I was hopping github will add a snippet to that comment)

[nicoddemus]

Nice work!

[nicoddemus]

(Note this is still marked as draft)