Add Pitch Register

[craigthomas]

This PR adds a pitch register to the CPU, as well as performs a playback rate calculation for the audio buffer. A new instruction for Fx3A is added called load_pitch that loads a pitch value from a V register into the pitch register. The new playback rate is calculated once the register is loaded with a new pitch. Unit tests added to cover new conditions. This PR closes #31

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.27%. Comparing base (52925f3) to head (1da3918).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   90.08%   90.27%   +0.18%     
==========================================
  Files           4        4              
  Lines         464      473       +9     
==========================================
+ Hits          418      427       +9     
  Misses         46       46              

Files	Coverage Δ
chip8/cpu.py	97.33% <100.00%> (+0.06%)	⬆️

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