Merge black constaints in the main requirement

requirements-conda.txt

 # Conda requirements file
+black == 18.6b4  # Dependency with ptpython, BLISS test, BLISS dev. Better to manage the constraint here
 aiogevent >= 0.4
 blessings
 cerberus

requirements-dev.txt

-black==18.6b4
+# Also used by the CI to check source code with black and flake
+
+black == 18.6b4
 flake8

requirements-test-conda-win64.txt

 # Conda requirement file
 # Test requirements
 
-black == 18.6b4
 lima-core >= 1.9.6
 lima-camera-simulator >= 1.9.2
 lima-camera-simulator-tango >= 1.9.2

requirements-test-conda.txt

 # Conda requirement file
 # Test requirements
 
-black == 18.6b4
 coilmq
 lima-core >= 1.9.6
 lima-camera-simulator >= 1.9.2