Brevo: Rename SendinBlue to Brevo

- Replace "SendinBlue" with "Brevo"
  throughout the code.
- Maintain deprecated compatibility
  versions on the old names/URLs.
  (Split into separate commit
  to make renamed files more
  obvious.)
- Update docs to reflect change,
  provide migration advice.
- Update integration workflow.

tox.ini

@@ -59,6 +59,7 @@ setenv =
     # (resend should work with or without its extras, so it isn't in `none`)
     none: ANYMAIL_SKIP_TESTS=amazon_ses,postal
     amazon_ses: ANYMAIL_ONLY_TEST=amazon_ses
+    brevo: ANYMAIL_ONLY_TEST=brevo
     mailersend: ANYMAIL_ONLY_TEST=mailersend
     mailgun: ANYMAIL_ONLY_TEST=mailgun
     mailjet: ANYMAIL_ONLY_TEST=mailjet
@@ -68,7 +69,6 @@ setenv =
     resend: ANYMAIL_ONLY_TEST=resend
     sendgrid: ANYMAIL_ONLY_TEST=sendgrid
     unisender_go: ANYMAIL_ONLY_TEST=unisender_go
-    sendinblue: ANYMAIL_ONLY_TEST=sendinblue
     sparkpost: ANYMAIL_ONLY_TEST=sparkpost
 ignore_outcome =
     # CI that wants to handle errors itself can set TOX_OVERRIDE_IGNORE_OUTCOME=false