Fix ryy gate definition

[bramathon]

Description

• Fixes the RYY gate definition RYY gate definition is wrong #1602 [auto-close]
• Addes a new function in pyquil.paulis called exponentiate_pauli_sum. This function performs exponentiation of non-commuting PauliSums, but only for PauliSum with numeric values. The return is a unitary.
• Tests that exponentiate_pauli_sum produces the quil gates.

Checklist

• The PR targets the master branch
• The above description motivates these changes.
• The change is atomic and can be described by a single commit (your PR will be squashed on merge).
• All changes to code are covered via unit tests.
• Parameters and return values have type hints with [PEP 484 syntax][pep-484].
• Functions and classes have useful [Sphinx-style][sphinx] docstrings.
• (New Feature) The [docs][docs] have been updated accordingly.
• (Bugfix) The associated issue is referenced above using [auto-close keywords][auto-close].

[bramathon]

I don't know how to fix this error:

pyquil/paulis.py:986: error: Call to untyped function "real" in typed context
pyquil/paulis.py:989: error: Returning Any from function declared to return "ndarray[Any, Any]"
pyquil/paulis.py:989: error: Call to untyped function "angle" in typed context

I guess it means that the numpy function np.angle isn't typed.