Define flake8 in requirements_test.txt

Signed-off-by: Beat Buesser <[email protected]>

Author: Beat Buesser

requirements_test.txt

@@ -53,6 +53,7 @@ jax[cpu]==0.3.15
 # other
 pytest~=7.1.2
 pytest-flake8~=1.1.1
+flake8~=4.0.1
 pytest-mock~=3.8.2
 pytest-cov~=3.0.0
 codecov~=2.1.12