test: py: add pygit2 and pyelftools to requirements.txt

Add pygit2 and pyelftools to the list of packages for virtualenv
needed to run all sets of pytests.This fixes warnings like:

binman.elf_test.TestElf.testDecodeElf (subunit.RemotedTestCase):
Python elftools not available

Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Reviewed-by: Simon Glass <sjg@chromium.org>