Add support for job scheduling

[sdispater]

This PR adds two new configuration option for jobs: schedule_at and schedule_in.

• schedule_at must be a timezone-aware datetime representing the moment the job must be executed.
• schedule_in must be a dictionnary representing the relative time the job must be executed at

import pendulum
from cabbage import TaskManager

manager = TaskManager()


@manager.task(queue="default")
def add(a, b):
    return a + b

add.configure(schedule_at=pendulum.now().add(minutes=2)).defer(a=1, b=2)
add.configure(schedule_in={"minutes": 2}).defer(a=1, b=2)

[ewjoachim]

add pendulum.* at https://github.com/peopledoc/cabbage/pull/26/files#diff-380c6a8ebbbce17d55d50ef17d3cf906R61 to please mypy.

(and maybe add type annotations in a compatible way as decribed in https://mypy.readthedocs.io/en/latest/installed_packages.html#making-pep-561-compatible-packages?)

[sdispater]

I updated the setup.cfg file.

And regarding type annotations, there is a PR (python-pendulum/pendulum#320) that I need to merge at some point.

[codecov-io]

Codecov Report

Merging #26 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #26   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          10     10           
  Lines         292    309   +17     
  Branches       12     17    +5     
=====================================
+ Hits          292    309   +17

Impacted Files	Coverage Δ
cabbage/postgres.py	100% <ø> (ø)	⬆️
cabbage/tasks.py	100% <100%> (ø)	⬆️
cabbage/jobs.py	100% <100%> (ø)	⬆️
cabbage/testing.py	100% <100%> (ø)	⬆️

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