Drop Cython

[paulmueller]

Since we might have https://github.com/tsbischof/libpicoquant as a Python module at some point, we could possibly drop the built-in read_pt3_PicoQuant.py reader which will free PyCorrFit from its Cython dependency. Creating builds will be much easier with a pure-Python application.

This affects #201 #202 #203 equally.

Just for reference:

• An updated version of the pt3 file reader currently copy-pasted in PyCorrFit: ref: use basic code from FCS point correlator #196
• Request for .ptu file format: Picoquant 64-bit .ptu file support #84

@tsbischof You said you planned to eventually create a libpicoquant Python package. Does your software also process the .pt3 files that PyCorrFit can currently read (from the docs it seems to be the case)? If yes, then I would probably keep all of the above issues on hold, because it would massively simplify creating a CI build pipeline.