added P gate to list of available gates that can be parsed from QASM

[googlercolin]

Summary

P gate was previously unable to be parsed from QASM to Cirq circuit. Added ability for p(params[0]) gates to be parsed as QasmUGate(0, 0, params[0] / np.pi).

Details & Comments

• Previously not able to parse P gate using circuit_from_qasm(qasm) method under cirq.contrib.qasm_import module.
• Added P gate into qelib_gates dictionary under _parser.py.

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[googlercolin]

It seemed like the format check failed. I ran the python code through black to match the format required.