make massmail installable using more modern techniques:
- pyproject.toml (supports editable installation since at least
  setuptools 66.1.1, but maybe even earlier)
- automatically generate massmail script on installation

all in one commit because the rewrite was not functional in the
development stages:

- use click for the command line interface [this makes for a much more
  consistent and clear CLI]

- port to the modern interface of the email stdlib module [takes care
  automatically of encoding issues and it is way more pythonic]

- do not use stdin anymore for the body, we use required options for
  both the parameter file and the body text: no more confusion about
  which is which like in the old interface!

- line count cut by almost half :-)))

TODO:

- write a callback for checking that CLI arguments that should
  be email address are indeed email addresses!
- rewrite the tests so that they work and add more tests
- detect use of stdin -> this means we are called using the old
  interace: we can generate a good error message!

...we make a heavy use of the pytest fixture system, which makes for
much clearer and compact test code

…d-mail logic

…email body

... get_content is documented, but does not automatically handle decoding of 8bit transferred
emails. get_payload is not documented, but is used internally by the email module and it handles
correctly the decoding...

… of the email body"

This commit was based on some wrong assumptions. Actually get_content
should always take care of the proper transfer encoding. If it doesn't
there is a bug in the email module and we shouldn't try to work around
it here.

This reverts commit 3155974.

… the code

…ed error

…f the csv file