akvo / siwins / #763

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Pull #209

coveralls-python

[#208] Send mail over SMTP instead of through Mailjet

Seed/sync notification mail went out through the Mailjet v3 REST API,
and the account stopped delivering: Mailjet accepted every message and
delivered none, while credentials, sender validation, SPF and DKIM all
reported healthy. Replace the client with `smtplib` and
`email.message.EmailMessage` from the standard library. `mailjet-rest`
is dropped from the requirements; `requests` is pinned directly, so
nothing else loses a transitive dependency.

The new settings are `EMAIL_HOST`, `EMAIL_PORT`, `EMAIL_HOST_USER`,
`EMAIL_HOST_PASSWORD`, `EMAIL_USE_TLS`, `EMAIL_USE_SSL` and
`EMAIL_FROM`. The names match the Django projects at
akvo/african-bamboo-dashboard and akvo/akvo-mis so that deployment
wiring stays consistent across repositories, even though there is no
framework mail backend here to plug them into.

Several choices are worth spelling out.

Every setting is read with `os.environ.get`. Mailjet's credentials were
read with `os.environ[...]` at import time, so a backend missing them
would not start at all; now development and test environments boot
without a relay configured, and a missing host is reported when
something actually tries to send.

The fallbacks are written as

    EMAIL_PORT = int(os.environ.get("EMAIL_PORT") or 587)

rather than as a `get` default, because Compose passes
`- EMAIL_PORT=${EMAIL_PORT}` through as an empty string when the
variable is unset in the shell. That value is present as far as `get`
is concerned, which would make `int("")` raise and would read an empty
`EMAIL_USE_TLS` as "not true" — quietly dropping the connection to
plaintext.

Defaults are those of an ordinary submission relay: port 587 with
STARTTLS on and implicit SSL off, so a deployment supplying only a host
and credentials lands on the common correct configuration. Neither TLS
mode is inferable from the port, so a relay on 465 must set both
`EMAIL_USE_SSL` and `EMAIL_USE_TLS` explicitly; with the wron... (continued)

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