The conventional approach for choosing an interpreter is based on
virtual environments.
Thus `/usr/bin/env python3` should be used instead of mixed shell/python
code for selecting a hard-coded interpreter.

See #102

Currently the database session is created during application startup and
is used in all threads.  This is problematic, since the session is not
thread-safe.
Now we use the request-based database session provided by
flask-sqlachemy instead.

But this does not really solve the problem of using the same session in
different threads, since the threads are started within a request.

Thanks, codespell!