inform users Python3.5 will be dropped (#1224)

CMakeLists.txt

@@ -685,6 +685,8 @@ if(NRN_ENABLE_PYTHON)
   message(STATUS "  DYNAMIC     | ${NRN_ENABLE_PYTHON_DYNAMIC}")
   if(PYTHON_VERSION_MAJOR EQUAL 2)
     set(PY2_WARNING TRUE)
+  elseif(PYTHON_VERSION_MINOR EQUAL 5)
+    set(PY35_WARNING TRUE)
   endif()
   if(NRN_ENABLE_PYTHON_DYNAMIC)
     list(LENGTH NRN_PYTHON_EXE_LIST _num_pythons)
@@ -694,6 +696,8 @@ if(NRN_ENABLE_PYTHON)
       list(GET NRN_PYTHON_VER_LIST ${val} version)
       if(version EQUAL 2)
         set(PY2_WARNING TRUE)
+      elseif(PYTHON_VERSION_MINOR EQUAL 5)
+        set(PY35_WARNING TRUE)
       endif()
       list(GET NRN_PYTHON_INCLUDE_LIST ${val} include)
       list(GET NRN_PYTHON_LIB_LIST ${val} lib)
@@ -745,8 +749,11 @@ message(STATUS " See documentation : https://www.neuron.yale.edu/neuron/")
 message(STATUS "--------------+--------------------------------------------------------------")
 message(STATUS "")
 if(PY2_WARNING)
-  message(WARNING "Python2 support will be dropped soon! We strongly advise migrating to Python3 as of now.")
+  message(WARNING "Python2 support will be dropped soon! We strongly advise migrating to Python3 (>=3.6) as of now.")
   if(NOT PATHLIB_FOUND)
     message(WARNING "'pathlib' Python2 package is required for in-python neuron documentation. Please install it.")
   endif()
 endif()
+if(PY35_WARNING)
+  message(WARNING "Python3.5 support will be dropped soon! We strongly advise migrating to a higher version as of now.")
+endif()

configure.ac

@@ -712,6 +712,9 @@ AC_MSG_WARN([
         * Python 2  : Please note that Python2 support will be DROPPED soon.
                       We strongly advise migrating to Python3 as of now,
                       if you haven't already.
+        * Python 3.5: Having reached end of support in 2020, Python 3.5 support
+                      will also be dropped soon. Should you be switching from
+                      Python2 please consider a higher version than 3.5.
         * Autotools : CMake is now the primary build system for installing NEURON
                       from source. Autotools support will be DROPPED in the next
                       release. As you are using Autotools, we strongly recommend