Release process updates and sourmash 3.0 release notes

[luizirber]

• Release process updates from 2.3.0
• Release note for 3.0.0

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 #776 into master will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #776      +/-   ##
==========================================
- Coverage   79.17%   78.93%   -0.25%     
==========================================
  Files          45       45              
  Lines        6695     6694       -1     
  Branches      470      470              
==========================================
- Hits         5301     5284      -17     
- Misses       1095     1111      +16     
  Partials      299      299

Flag	Coverage Δ
#pytests	90.36% <ø> (ø)	⬆️
#rusttests	48.73% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
sourmash/_compat.py	50% <0%> (-50%)	⬇️
sourmash/signature_json.py	96.21% <0%> (-1.09%)	⬇️
sourmash/lca/lca_utils.py	96.14% <0%> (-0.65%)	⬇️
sourmash/lca/command_index.py	91.02% <0%> (-0.41%)	⬇️
sourmash/sbt.py	86.32% <0%> (-0.3%)	⬇️
sourmash/sbt_storage.py	86.66% <0%> (+0.95%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3e71e9a...f8d8f48. Read the comment docs.