Support Debian's sensible-editor

[jwodder]

Debian and derivative distros include a program named sensible-editor that is similar in spirit to Editor.get_editor(), except it also allows the user to make an interactive choice when the relevant envvars are unset:

• It invokes $VISUAL or $EDITOR if they are defined
• Otherwise, the program specified in ~/.selected_editor is run. If this file does not exist and input is a tty, the user is shown a list of installed text editors and asked to choose one. The choice is saved in ~/.selected_editor, and then the given program is run.
• If the file does not exist and input is not a tty, or if the program saved in ~/.selected_editor no longer exists, sensible-editor falls back to nano, then nano-tiny, then vi.

This patch adds sensible-editor to the list of editors considered by Editor.get_editor() so that determining an editor to use on Debian systems works similarly to other programs on Debian.

[davidism]

Meant to target this at 7.1, didn't notice it was on master. Cherry picked in 17cc8d7.