Downgrade LIMITED_API_VERSION, if python3 is older than 3.6.

cmake/OpenCVDetectPython.cmake

@@ -295,7 +295,12 @@ find_python("${OPENCV_PYTHON3_VERSION}" "${MIN_VER_PYTHON3}" PYTHON3_LIBRARY PYT
 OCV_OPTION(PYTHON3_LIMITED_API "Build with Python Limited API (not available with numpy >=1.15 <1.17)" NO
            VISIBLE_IF PYTHON3_NUMPY_VERSION VERSION_LESS "1.15" OR NOT PYTHON3_NUMPY_VERSION VERSION_LESS "1.17")
 if(PYTHON3_LIMITED_API)
-  set(PYTHON3_LIMITED_API_VERSION "0x03060000" CACHE STRING "Minimal Python version for Limited API")
+  set(_default_ver "0x03060000")
+  if(PYTHON3_VERSION_STRING VERSION_LESS "3.6")
+    # fix for older pythons
+    set(_default_ver "0x030${PYTHON3_VERSION_MINOR}0000")
+  endif()
+  set(PYTHON3_LIMITED_API_VERSION ${_default_ver} CACHE STRING "Minimal Python version for Limited API")
 endif()
 
 if(PYTHON_DEFAULT_EXECUTABLE)