[MRG] deprecate import sourmash_lib

[ctb]

Fixes #441.

As in #1128 (comment), I used FutureWarning instead of DeprecationWarning so it would show up without any special configuration.

• 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 #1143 into master will increase coverage by 9.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1143      +/-   ##
==========================================
+ Coverage   83.88%   93.03%   +9.14%     
==========================================
  Files          98       74      -24     
  Lines        9124     5812    -3312     
==========================================
- Hits         7654     5407    -2247     
+ Misses       1470      405    -1065     

Flag	Coverage Δ
#rusttests	?

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Impacted Files	Coverage Δ
sourmash_lib/__init__.py	100.00% <100.00%> (ø)
src/core/src/signature.rs
src/core/src/index/mod.rs
src/core/src/sketch/minhash.rs
src/core/src/cmd.rs
src/core/src/index/search.rs
src/core/src/ffi/utils.rs
src/core/src/errors.rs
src/core/tests/test.rs
src/core/src/lib.rs
... and 15 more

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