Add return type annotations to api

[ionite34]

Closes #213

[codecov-commenter]

Codecov Report

Base: 97.79% // Head: 97.77% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (8ae7b2c) compared to base (5692862).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #214      +/-   ##
==========================================
- Coverage   97.79%   97.77%   -0.02%     
==========================================
  Files           7        6       -1     
  Lines         363      360       -3     
==========================================
- Hits          355      352       -3     
  Misses          8        8              

Impacted Files	Coverage Δ
yaspin/api.py	100.00% <100.00%> (ø)
yaspin/__init__.py

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[pavdmyt]

LGTM; @ionite34 thanks for your contribution!