[AIRFLOW-1958] Add **kwargs to send_email

[ms32035]

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-1958

Description

• **kwargs added to email function definitions. This can be used with custom backends

Tests

• Covered by existing cases (at least should be)

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":

  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[codecov-io]

Codecov Report

Merging #2908 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2908   +/-   ##
=======================================
  Coverage   73.27%   73.27%           
=======================================
  Files         160      160           
  Lines       12196    12196           
=======================================
  Hits         8937     8937           
  Misses       3259     3259

Impacted Files	Coverage Δ
airflow/utils/email.py	97.4% <100%> (ø)	⬆️

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[fenglu-db]

LGTM

[ms32035]

can this be merged? I plan to expand the sendgrid backend basing on this

[fenglu-db]

@criccomini could you help on merging this PR?

[criccomini]

lgtm!