Deprecate imp [Breaks py2.7 compatibility]

[sergioisidoro]

Getting the following warnings:

venv/lib/python3.7/site-packages/flasgger/utils.py:5
 
venv/lib/python3.7/site-packages/flasgger/utils.py:5: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
 
    import imp

-- Docs: https://docs.pytest.org/en/latest/warnings.html

I guess this relates to #399 , but I think since Python2 has officially reached the EOL, this makes sense.

This makes this package incompatible with Python < 3.3