[WIP] Fixes for asv benchmarks

[luizirber]

In #1571 @mr-eyes mentioned he couldn't run the asv benchmarks. I also had problems fixing them for #1564. It seems that one of the main issues is that virtualenv was using a very old version (because asv had issues with newer versions), but it seemed to be fixed and it is working with newer virtualenv again).

This PR avoid pinning the virtualenv version, and is enough to make it work in my system.

@mr-eyes, can you try this branch and see if benchmarks (with tox -e asv) work?

[codecov]

Codecov Report

Merging #1576 (828e89f) into latest (6b5806c) will increase coverage by 8.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1576      +/-   ##
==========================================
+ Coverage   80.98%   89.11%   +8.12%     
==========================================
  Files         102       75      -27     
  Lines       10331     6623    -3708     
  Branches     1171     1171              
==========================================
- Hits         8367     5902    -2465     
+ Misses       1758      515    -1243     
  Partials      206      206              

Flag	Coverage Δ
python	89.11% <ø> (ø)
rust	?

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Impacted Files	Coverage Δ
src/core/src/ffi/signature.rs
src/core/src/sketch/nodegraph.rs
src/core/src/ffi/minhash.rs
src/core/src/signature.rs
src/core/src/from.rs
src/core/src/ffi/nodegraph.rs
src/core/tests/test.rs
src/core/src/wasm.rs
src/core/src/index/sbt/mod.rs
src/core/src/index/storage.rs
... and 17 more

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[mr-eyes]

Thanks, Luiz. I want to clarify the issue that I was experiencing.

When I tried tox -e asv on latest it was working fine. But when I add new functions to benchmarks.py I always get an error stating that benchmarking is getting slow due to the new functions.

Another error I got is that benchmarking is always set to run on Python 3.7, and actually, the sourmash_dev environment does not contain that version of Python. So I changed the asv.conf.json: Pythons to 3.9.

Update: Sometimes I had to remove the .tox directory to get it working.

[mr-eyes]

I would like also to suggest adding instructions for the benchmarking in the Sourmash dev docs section.
Including but not limited to:

• What is benchmarks.py and how it's working?
• How to add new functions to benchmark?

[luizirber]

I think this was fixed in #1834