Update requirements.txt

remove imageio == *
skimage => scikit-image
tensorflow-gpu => tensorflow-gpu == 1.*  Because tensorflow.contrib is removed in tensorflow 2

So that it can be used via pip install -r requirements.txt

requirements.txt

@@ -1,8 +1,7 @@
-jupyterlab = "*"
-tensorflow-gpu = "*"
-imageio = "*"
-matplotlib = "*"
-scipy = "*"
-numpy = "*"
-skimage = "*"
-imageio == 2.4.0
+jupyterlab
+tensorflow-gpu == 1.*
+matplotlib
+scipy
+numpy
+scikit-image
+imageio == 2.4.0