Move unused concepts into a `fragments` subdir
Add stuff about sys.stdout, stdin, stderr, error codes

bottom-up vs top-down

airport example

Just becuse

23 pages done, 20 to go

* Use lstlisting instead of minted
* Only use ASCII letters in code samples (sigh ...)

too many students have Python < 3.6

Just in case session 1 is too short

Do not talk about 'continue' just yet