Leave conda dependencies floating instead of pinned

[savingoyal]

@conda currently pins some dependencies to specific versions (requests, click etc.). This can cause conflict with some other user-defined packages (snowflake-client etc.) which might pin the same packages to a different version. Also, anytime Metaflow updates the package versions, it ends up resolving the user environment again, which is undesirable. We should leave the dependencies floating.

[bishax]

@savingoyal I'm running into this problem on python 3.9 with no deps declared because the pinned version of coverage (4.5.4) isn't available for 3.9 (this means any newcomer on 3.9 trying metaflow with the conda environment will hit an Unsatisfiable environment error!)

$ conda search coverage | grep py39
coverage                         5.3  py39h07f9747_1  conda-forge         
coverage                         5.3  py39h27cfd23_2  pkgs/main           
coverage                         5.3  py39h3811e60_1  conda-forge         
coverage                       5.3.1  py39h27cfd23_2  pkgs/main           
coverage                       5.3.1  py39h3811e60_0  conda-forge         
coverage                       5.3.1  py39h3811e60_1  conda-forge         
coverage                         5.4  py39h27cfd23_2  pkgs/main           
coverage                         5.4  py39h3811e60_0  conda-forge         
coverage                         5.5  py39h27cfd23_2  pkgs/main           
coverage                         5.5  py39h3811e60_0  conda-forge

Minus some formatting and commenting then #411 looks like it would close this issue.
Did it just slip off the backlog or was there a reason for not merging?
If it's the first I can revive that PR (or @rchui can if they'd prefer)?

[rchui]

@bishax The branch fell way behind and slipped off the backlog for the metaflow team. You're welcome to revive it and give it another try.

[savingoyal]

addressed in #1505