remove setuptools from install_requires (#118)

pkg_resources is no longer used

setup.py

@@ -58,7 +58,7 @@
     provides=["imageio_ffmpeg"],
     python_requires=">=3.5",
     setup_requires=[],
-    install_requires=["setuptools"],
+    install_requires=[],
     packages=["imageio_ffmpeg", "imageio_ffmpeg.binaries"],
     package_data={"imageio_ffmpeg.binaries": ["*.*"]},
     include_package_data=True,