insane.bbclass: make sure to close .patch files

* fixes:
  DEBUG: Executing python function do_qa_patch
  /OE/build/oe-core/openembedded-core/meta/classes/insane.bbclass:1189: ResourceWarning: unclosed file <_io.TextIOWrapper name='/OE/build/oe-core/openembedded-core/meta/recipes-bsp/keymaps/files/GPLv2.patch' mode='r' encoding='utf-8'>
    content = open(fullpath, encoding='utf-8', errors='ignore').read()
  ResourceWarning: Enable tracemalloc to get the object allocation traceback
  DEBUG: Python function do_qa_patch finished

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>