ENH: Patch for python 3.8

[gerrymanoim]

Patch for 3.8. Doesn't actually add support for posonlyargcount, just makes it so code doesn't crash.

Closes https://github.com/llllllllll/codetransformer/issues/72

There's still some test failures on 3.8 that are a bit confusing, but I believe the functionality itself works.

@llllllllll - I'm not sure if this fix is what you were describing in https://github.com/llllllllll/codetransformer/issues/72#issuecomment-584981635