Appease sphinx by importing quilatom.Expression in gates.py

Import quilatom.Expression in gates.py. Even though it isn't
referenced directly, it is indirectly used by
quilatom.ParameterDesignator, which causes sphinx to choke when
generating the docs.

pyquil/gates.py

@@ -16,9 +16,9 @@
 from warnings import warn
 from typing import Any, Callable, Dict, Optional, Tuple, Union
 
-from pyquil.quilatom import (Addr, MemoryReference, MemoryReferenceDesignator, Parameter,
-                             ParameterDesignator, Qubit, QubitDesignator, QubitPlaceholder,
-                             unpack_classical_reg, unpack_qubit)
+from pyquil.quilatom import (Addr, Expression, MemoryReference, MemoryReferenceDesignator,
+                             Parameter, ParameterDesignator, Qubit, QubitDesignator,
+                             QubitPlaceholder, unpack_classical_reg, unpack_qubit)
 from pyquil.quilbase import (AbstractInstruction, Gate, Halt, Reset, ResetQubit, Measurement, Nop,
                              Wait,
                              ClassicalNeg, ClassicalNot,