Fix build/CI and unused imports papercuts

[luizirber]

• remove unused imports in src/sourmash/search.py
• add a mypy check task to tox (still need to be executed explicitly)
• set up dependabot for python, rust and gh action updates

[codecov]

Codecov Report

Merging #1974 (9b94332) into latest (7433b7f) will increase coverage by 7.89%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1974      +/-   ##
==========================================
+ Coverage   83.54%   91.43%   +7.89%     
==========================================
  Files         127       96      -31     
  Lines       14235     9953    -4282     
  Branches     1946     1946              
==========================================
- Hits        11892     9101    -2791     
+ Misses       2070      579    -1491     
  Partials      273      273              

Flag	Coverage Δ
python	91.43% <ø> (-0.01%)	⬇️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/search.py	97.68% <ø> (-0.02%)	⬇️
src/core/src/ffi/utils.rs
src/core/src/ffi/index/mod.rs
src/core/src/ffi/mod.rs
src/core/src/sketch/hyperloglog/mod.rs
src/core/src/ffi/hyperloglog.rs
src/core/tests/minhash.rs
src/core/src/index/revindex.rs
src/core/src/index/linear.rs
src/core/src/index/sbt/mhbt.rs
... and 22 more

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[mr-eyes]

Great! and I will just point the mypy update here to #1844 for future reference.

[mr-eyes]

Seems like referencing is not working from the review comment so just mentioning again #1844