Log a warning when aqtinstall falls back to an external 7z extraction tool

[ddalcino]

Inspired by #696 (comment). That comment suggested a place to add a log message, but I was unable to log the message there without disregarding the user’s selected log settings, which are not available until the Cli object is built.

When aqtinstall fails to load the py7zr module, it falls back to whatever 7z extraction tool the user has specified in the settings.ini file, without notifying the user. This PR adds the following log message when that happens:

WARNING: The py7zr module failed to load. Falling back to '7z' for .7z extraction.
WARNING: You can use the  '--external | -E' flags to select your own extraction tool.

This PR also does a little refactoring of the Cli._set_sevenzip method to improve readability.

Side effect
There was an existing bug here, where the install-qt subcommand (but none of the other commands) would ignore the 7zcmd parameter in the settings.ini file, and use the hardcoded "7z" instead. I'm pretty sure this was a bug; if not I can change it back.

aqtinstall/aqt/installer.py

Lines 361 to 363 in 72da54f

	if EXT7Z and sevenzip is None:
	# override when py7zr is not exist
	sevenzip = self._set_sevenzip("7z")

[miurahr]

Thank you for the improvement.