user: Question: what's party with district being Pennsylvania 21
Columns: ['District', 'Incumbent', 'Party', 'First elected', 'Result', 'Candidates']
Types: ['text', 'text', 'text', 'real', 'text', 'text']
Sample row: ['Pennsylvania 6', 'Gus Yatron', 'Democratic', 1968.0, 'Re-elected', 'Gus Yatron (D) 69.1% Norm Bertasavage (R) 30.9%']
SQL:

assistant: SELECT "Party" FROM "Table" WHERE "District" = 'Pennsylvania 21';