[setup] Move import to FFi directly in Builder

[anth2o]

Hello,
I have an issue when trying to install your package:

python3 -m pip install git+https://github.com/QuantumPhotonicsLab/readPTU.git
Collecting git+https://github.com/QuantumPhotonicsLab/readPTU.git
  Cloning https://github.com/QuantumPhotonicsLab/readPTU.git to /private/var/folders/r7/wn2zm4pd5g137j4wf12279k80000gn/T/pip-req-build-l2m3kdor
  Running command git clone -q https://github.com/QuantumPhotonicsLab/readPTU.git /private/var/folders/r7/wn2zm4pd5g137j4wf12279k80000gn/T/pip-req-build-l2m3kdor
    ERROR: Command errored out with exit status 1:
     command: /Users/antoine/movexplo/env/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/var/folders/r7/wn2zm4pd5g137j4wf12279k80000gn/T/pip-req-build-l2m3kdor/setup.py'"'"'; __file__='"'"'/private/var/folders/r7/wn2zm4pd5g137j4wf12279k80000gn/T/pip-req-build-l2m3kdor/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /private/var/folders/r7/wn2zm4pd5g137j4wf12279k80000gn/T/pip-req-build-l2m3kdor/pip-egg-info
         cwd: /private/var/folders/r7/wn2zm4pd5g137j4wf12279k80000gn/T/pip-req-build-l2m3kdor/
    Complete output (5 lines):
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/private/var/folders/r7/wn2zm4pd5g137j4wf12279k80000gn/T/pip-req-build-l2m3kdor/setup.py", line 4, in <module>
        from cffi import FFI
    ModuleNotFoundError: No module named 'cffi'
    ----------------------------------------

This PR should solve this by moving the import in the Builder. I made sure my solution worked by executing pip install . without cffi already installed

I also modified the setup.py to move the compile operations from the install to the build step, so that pip install . also works.