Add python type annotations

[SamWilsn]

Not 100% sure it's safe to use ReadableBuffer. I'm guessing that .str = data makes a copy, unlike ffi.from_buffer?

[codecov[bot]]

Codecov Report

Merging #234 (7457a14) into master (3f74b37) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #234   +/-   ##
=======================================
  Coverage   98.65%   98.65%           
=======================================
  Files          17       17           
  Lines        1337     1337           
=======================================
  Hits         1319     1319           
  Misses         18       18           

Flag	Coverage Δ
be	85.35% <ø> (ø)
default	98.41% <ø> (ø)
x86_64	85.03% <ø> (ø)

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