[MRG] Dev updates (configs and doc)

[luizirber]

I need the shell.nix now that both my dev machines are running NixOS. This PR updates an earlier version, adding all the supported Python versions and using niv for versioning/reproducibility of the environment.

TODO

• add instructions to dev documentation (also add tox-conda info?)
• Add CI for checking if the environment is working? (only py39 tests)
• Also add CI for tox-conda? (only py39 tests)

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1298 (035c5d7) into latest (43ae278) will increase coverage by 5.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1298      +/-   ##
==========================================
+ Coverage   88.69%   93.96%   +5.26%     
==========================================
  Files         125       98      -27     
  Lines       18281    14665    -3616     
  Branches     1440     1440              
==========================================
- Hits        16215    13780    -2435     
+ Misses       1819      638    -1181     
  Partials      247      247              

Flag	Coverage Δ
python	93.96% <ø> (ø)
rust	?

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Impacted Files	Coverage Δ
src/core/tests/minhash.rs
src/core/src/index/linear.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/index/storage.rs
src/core/src/sketch/minhash.rs
src/core/src/sketch/nodegraph.rs
src/core/src/wasm.rs
src/core/src/index/sbt/mod.rs
src/core/src/ffi/nodegraph.rs
src/core/src/ffi/cmd/compute.rs
... and 17 more

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[luizirber]

I think this is ready for review and merge. Do the new install instructions make sense?

Pinging @ctb @olgabot @bluegenes on this one =]