This patch updates the dependency on blitiri.com.ar/go/spf from v1.2.0
to v1.3.0, which includes a few bug fixes.
There are no code changes needed, just some minor adjustment to the
tests due to error strings changing.
The go.mod "go" keyword is also bumped up to 1.15 since it's the minimum
supported version since commit e444fe1f (2021-10-05).
This patch adds a new integration test to cover SPF checks. The main
goal is not to cover the SPF parsing, since that's handled by the
library already, but the higher level aspects: that the mails are indeed
rejected, that the DSN looks reasonable, etc.
