High pitched sounds out of tune
First off, thanks for all your hard work on Pyxel. It is a fantastic package!

**Describe the bug**
When using the music editor, I have noticed that the high pitched notes do not seem to be in tune. For example, if I play each octave of D, the lower ones sound correct but the highest is out of tune.

**To Reproduce**
Open the music editor and play the same note at different octaves including the highest.

**Expected behavior**
All instances of the same note should sound in tune.

**Desktop (please complete the following information):**
 - OS: macOS 10.14.5 and Windows 10

If you have any suggestions as to what might be causing this, I am happy to play around with the source code and submit a pull request if I manage to fix the problem.

Thanks again!