move back to requirements for python3.6

requirements-3.7.txt

+# If you're changing this, consider relaunching:
+# https://gricad-gitlab.univ-grenoble-alpes.fr/ttk/docker-ttk/-/pipelines
+
+numpy==1.20.1 # Python >=3.7
+
+scipy==1.6.1 # Python >=3.7
+
+scikit-image==0.18.1 # Python >=3.7
+
+tifffile==2021.3.16 # Python >=3.7
+
+matplotlib==3.3.4 # Python >=3.6
+
+SimpleITK==2.0.2
+
+progressbar==2.5
+
+gmsh==4.8.0
+meshio==4.3.11 # Python >=3.6
+
+# Anything later than this causes an abort at the end of sphinx 3.5.2 build
+pybind11==2.5.0 # Python !=3.0, !=3.1, !=3.2, !=3.3, !=3.4, >=2.7
+
+# multigrid solver for random walker segmentation
+pyamg==4.0.0
+
+############
+# Optional #
+############
+
+# mpi
+# mpi4py==3.0.3
+
+# Jupyter
+#ipykernel==5.5.0 # Python >=3.5
+ipython==7.21.0 # Python >=3.7
+
+# R (correlated random field)
+rpy2==3.2.2
+
+#############
+# Graphical #
+#############
+#PyQt5==5.15.4 # Python >=3.6
+#PyQt5-sip==12.8.1 # Python >=3.5
+#qimage2ndarray==1.8.3

requirements.txt

 # If you're changing this, consider relaunching:
 # https://gricad-gitlab.univ-grenoble-alpes.fr/ttk/docker-ttk/-/pipelines
 
-numpy==1.20.1 # Python >=3.7
+numpy==1.19.5 # Python >=3.6
 
-scipy==1.6.1 # Python >=3.7
+scipy==1.5.4 # Python >=3.6
 
-scikit-image==0.18.1 # Python >=3.7
+scikit-image==0.17.2 # Python >=3.6
 
-tifffile==2021.3.16 # Python >=3.7
+tifffile==2020.8.25 # Python >=3.6
 
 matplotlib==3.3.4 # Python >=3.6
 
@@ -33,7 +33,7 @@ pyamg==4.0.0
 
 # Jupyter
 #ipykernel==5.5.0 # Python >=3.5
-ipython==7.21.0 # Python >=3.7
+ipython==7.16.1 # Python >=3.6
 
 # R (correlated random field)
 rpy2==3.2.2