`.add_foreign_keys()` doesn't reject being called with a View

[simonw]

Spotted this bug using mypy while working on #311 / #312!

% mypy sqlite_utils
sqlite_utils/db.py:725: error: Item "View" of "Union[Table, View]" has no attribute "foreign_keys"
Found 1 error in 1 file (checked 5 source files)

Refers to this code:

sqlite-utils/sqlite_utils/db.py

Lines 710 to 720 in c11ff89

	# We will silently skip foreign keys that exist already
	if not any(
	fk
	for fk in self[table].foreign_keys
	if fk.column == column
	and fk.other_table == other_table
	and fk.other_column == other_column
	):
	foreign_keys_to_create.append(
	(table, column, other_table, other_column)
	)

It's a bug! We run some checks earlier but none of them ensure that it's a view:

sqlite-utils/sqlite_utils/db.py

Lines 697 to 709 in c11ff89

	if not self[table].exists():
	raise AlterError("No such table: {}".format(table))
	if column not in self[table].columns_dict:
	raise AlterError("No such column: {} in {}".format(column, table))
	if not self[other_table].exists():
	raise AlterError("No such other_table: {}".format(other_table))
	if (
	other_column != "rowid"
	and other_column not in self[other_table].columns_dict
	):
	raise AlterError(
	"No such other_column: {} in {}".format(other_column, other_table)
	)