Update minumum Python version to 3.8.

Required to support Python 3.13 and later because the new C API used
in 1d8c525af was introduced in Python 3.8; the previous API was a
holdover from Python 2, deprecated in 3.7, and is removed in 3.13.

CMakeLists.txt

@@ -499,7 +499,7 @@ endif()
 # ############################################################
 
 if (WITH_PYTHON)
-  set (PYTHON_MIN_VERSION 3.6.0)
+  set (PYTHON_MIN_VERSION 3.8.0)
   if (NOT DEFINED Python3_FIND_UNVERSIONED_NAMES)
     set (Python3_FIND_UNVERSIONED_NAMES FIRST)
   endif()

README.dependencies

@@ -96,7 +96,7 @@ Libraries/Deps
   aqbanking             6.4.0                    online banking and SWIFT
   gwenhywfar            5.8.0                    file imports.
 
-  python                3.6.0                    python bindings; headers
+  python                3.8.0                    python bindings; headers
                                                  required, not just binaries.
   libofx                0.9.12                   OFX/QFX import