Deprecate Python 3.5

[Ezzer17]

On readthedocs it is stated that packages are compatible with Python 3.5+. However, Commit 0583c1e8 adds import of secrets module, which was added in Python 3.6. This makes python-engineio incompatible with Python 3.5. To avoid confusion, library documentation should probably be modified.

[miguelgrinberg]

Thanks, the references to Python 3.5 in the documentation are now removed.