gcc-changelog: remove file descriptor leaks

gcc-changelog: remove file descriptor leaks

Currently, running gcc-changelog's unit tests may clutter the output
with tons of warnings such as:

    .../contrib/gcc-changelog/git_email.py:40: ResourceWarning: unclosed
    file <_io.TextIOWrapper name='/tmp/tmpt5okd4qp.patch' mode='r'
    encoding='UTF-8'>
      lines = open(self.filename).read().splitlines()
    ResourceWarning: Enable tracemalloc to get the object allocation
    traceback

This commit fixes these leaks, which restores a clean testsuite output.

contrib/

	* gcc-changelog/git_update_version.py: Close file objects after
	use.
	* gcc-changelog/git_email.py: Likewise.
	* gcc-changelog/test_email.py: Likewise.