Add support for 'global-eldoc-mode' fix for Emacs 26+

Philip Blair requested to merge peblair/python-mode:emacs-26-fix-eldoc into master

python-mode had an issue on Emacs 25 which caused the *Python-Help* window to frequently appear when placing the cursor next to a piece of text for a second or so. This was fixed by disabling global-eldoc-mode, but python-mode does this if the major version is exactly 25. This patch updates this functionality to support Emacs 26+ as well, since the issue is still present on Emacs 26.

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