traduction de email.message (#177)

Reviewed-on: https://git.afpy.org/AFPy/python-docs-fr/pulls/177
Reviewed-by: Julien Palard <[email protected]>
Co-authored-by: Christophe Nanteuil <[email protected]>
Co-committed-by: Christophe Nanteuil <[email protected]>

Author: christopheNan