Adds a column with the argument secret=True to Table._meta.secret_columns

[sinisaos]

Related to #918. Code written by @patrickneise (here) but never create PR. Thanks @patrickneise.

[dantownsend]

Thanks for this - it looks good, just a few minor comments 👍

[dantownsend]

I don't think we need to check isinstance(column, Secret) - just column._meta.secret alone should work, because Secret columns should have column._meta.secret = True.

[dantownsend]

As a sanity check, add a non-secret column here too, to make sure only secret columns are added to _meta.secret_columns. For example:

class Classified(Table):
    top_secret = Varchar(secret=True)
    public = Varchar()

You might also consider merging this test with the above test, so we have this:

def test_secret_columns(self):
    """
    Make sure ``TableMeta.secret_columns`` are setup correctly.
    """

    class Classified(Table):
        top_secret = Secret()
        confidential = Varchar(secret=True)
        public = Varchar()

    self.assertEqual(
        Classified._meta.secret_columns,
        [Classified.top_secret, Classified.confidential]
    )

[dantownsend]

This is good to go once the CI passes - thanks!