[MRG] Switch use of load_signatures over to load_file_as_signatures throughout.

[ctb]

Per #1296 (comment)

This fixes deprecation warnings caused by merge of #1279.

Note that, viz tests/test_api.py, we no longer provide a top-level / public API for loading signatures from strings.

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 #1304 (c3ec1e9) into latest (3e3cd35) will increase coverage by 5.25%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1304      +/-   ##
==========================================
+ Coverage   88.70%   93.96%   +5.25%     
==========================================
  Files         125       98      -27     
  Lines       18294    14680    -3614     
  Branches     1437     1437              
==========================================
- Hits        16228    13794    -2434     
+ Misses       1821      641    -1180     
  Partials      245      245              

Flag	Coverage Δ
python	93.96% <100.00%> (-0.01%)	⬇️
rust	?

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Impacted Files	Coverage Δ
tests/test_api.py	100.00% <100.00%> (ø)
tests/test_cmd_signature.py	100.00% <100.00%> (ø)
tests/test_compare.py	100.00% <100.00%> (ø)
tests/test_index.py	100.00% <100.00%> (ø)
tests/test_sbt.py	100.00% <100.00%> (ø)
src/sourmash/__init__.py	91.66% <0.00%> (-2.78%)	⬇️
src/core/src/signature.rs
src/core/src/sketch/minhash.rs
src/core/src/index/sbt/mhbt.rs
src/core/src/ffi/cmd/compute.rs
... and 23 more

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