Remove Cython from requirements.txt (#2604)

Cython should be a dependency of the remaining packages in requirements.txt, so should be installed anyway even if not a direct requirement.

requirements.txt

@@ -1,7 +1,6 @@
 # pip install -r requirements.txt
 
 # base ----------------------------------------
-Cython
 matplotlib>=3.2.2
 numpy>=1.18.5
 opencv-python>=4.1.2