feat(pg_dump): Add '--if-exists' option for pg_dump (and add tests)

[WillNilges]

Description

• Adds --if-exists option for pg_dump command
• Resolves dbrestore fails with non-existent relation #478
• Adds simple test

Extension of #511. If I understand correctly this was precluded on missing a test? I have added it, and was hoping to get this merged so I can use it in production 🙇🏻

Feel free to cherry pick my commit onto @DmytroLitvinov's PR or however you want to do it, I just wanted the feature to move 😅

Checklist

Please update this checklist as you complete each item:

• Tests have been developed for bug fixes or new functionality.
• The changelog has been updated, if necessary.
• Documentation has been updated, if necessary.
• GitHub Issues closed by this PR have been linked.

_{By submitting this pull request I agree that all contributions comply with this project's open source license(s).}

[DmytroLitvinov]

Hi @WillNilges ,
Thanks for the help 👍

Added your commit into my initial PR: #511

[Archmonger]

Just a heads up that this test seems to be broken.

[WillNilges]

Oops, that's my bad. Should work now.

[Archmonger]

Marking this as draft due to tests failing

[Archmonger]

The following error will need to be resolved

  Traceback (most recent call last):
    File "/home/runner/work/django-dbbackup/django-dbbackup/.tox/py38-django42/lib/python3.8/site-packages/gnupg.py", line 159, in _copy_data
      outstream.write(data)
  TypeError: a bytes-like object is required, not 'str'
  ....ERROR Exception: Foo
    File "/home/runner/work/django-dbbackup/django-dbbackup/dbbackup/utils.py", line 119, in wrapper
      func(*args, **kwargs)
    File "/home/runner/work/django-dbbackup/django-dbbackup/dbbackup/tests/test_utils.py", line 81, in func
      raise Exception("Foo")
  
  .ERROR Exception: Foo
    File "/home/runner/work/django-dbbackup/django-dbbackup/dbbackup/utils.py", line 119, in wrapper
      func(*args, **kwargs)
    File "/home/runner/work/django-dbbackup/django-dbbackup/dbbackup/tests/test_utils.py", line 91, in func
      raise Exception("Foo")
  
  ..ERROR Error sending data
  Traceback (most recent call last):
    File "/home/runner/work/django-dbbackup/django-dbbackup/.tox/py38-django42/lib/python3.8/site-packages/gnupg.py", line 159, in _copy_data
      outstream.write(data)
  TypeError: a bytes-like object is required, not 'str'

Somehow your changes broke test_raise_error_without_mail

    @patch("dbbackup.settings.SEND_EMAIL", False)
    def test_raise_error_without_mail(self):
        def func():
            raise Exception("Foo")

        with self.assertRaises(Exception):
            utils.email_uncaught_exception(func)()
        self.assertEqual(len(mail.outbox), 0)

Specifically, it seems like gnupg is trying to encrypt some values before sending the email but it is receiving a string instead of bytes.

Truthfully, I am not sure how your changes could have caused this.