Remove setup_requires from setup.py

[johnthagen]

Using setup_requires requires that pip has no way of controlling where these dependencies are located.

psycopg2cffi/setup.py

Lines 67 to 69 in 83234af

	setup_requires=[
	'cffi>=1.0',
	],

See

• Avoid using setup_requires parameter in setup.py. pytest-dev/pytest-xdist#136
• https://pip.pypa.io/en/stable/reference/pip_install/#controlling-setup-requires

This makes it difficult/impossible to bundle this library for offline installs, for example, using pip download.