Do not emit warnings from opflow initialisation

Opflow is completely deprecated, and `import qiskit.opflow` should and
does raise a warning blaming the importer.  However, during the import,
several opflow objects are initialised, which also trigger deprecation
warnings.  These will correctly be blamed on internal Qiskit library
code, and so will not be shown to users by default.

However, test suites running with all warnings treated as errors will
also see these warnings, which will mask the one true warning that
should be handled by the downstream code.  It would be valid for
downstream code to filter out these internal warnings, but in order to
make it easier for downstream code to do the correct thing immediately,
this opts to silence the internal warnings during initialisation.
Downstream code running with all warnings enabled will now _only_ see
the warning from `import qiskit.opflow` which is the only actionable
warning they should have to deal with anyway.

These warnings are only filtered during the initialisation and are
targetted specifically at opflow-internal deprecation warnings.  In
general, deprecated code will call other deprecated code and it is not
necessary to filter every single usage.  This particular commit is a
slight compromise to make it slightly easier for downstream libraries to
do the right thing.

qiskit/opflow/operator_globals.py

@@ -14,6 +14,8 @@
 Operator Globals
 """
 
+import warnings
+
 from qiskit.quantum_info import Pauli
 from qiskit.circuit.library import CXGate, SGate, TGate, HGate, SwapGate, CZGate
 
@@ -48,22 +50,30 @@ def make_immutable(obj):
     return obj
 
 
-# 1-Qubit Paulis
-X = make_immutable(PauliOp(Pauli("X")))
-Y = make_immutable(PauliOp(Pauli("Y")))
-Z = make_immutable(PauliOp(Pauli("Z")))
-I = make_immutable(PauliOp(Pauli("I")))
+# All the deprecation warnings triggered by these object creations correctly blame `qiskit.opflow`
+# and so are not shown to users by default. However, since they are eagerly triggered at `import
+# qiskit.opflow`, they obscure the one "true" warning of the import when downstream testing code is
+# running with all warnings showing.  The true warning that really needs attention becomes easy to
+# overlook because there's so many that the downstream code didn't explicitly call.
+with warnings.catch_warnings():
+    warnings.filterwarnings("ignore", category=DeprecationWarning, module=r"qiskit\.opflow\.")
+
+    # 1-Qubit Paulis
+    X = make_immutable(PauliOp(Pauli("X")))
+    Y = make_immutable(PauliOp(Pauli("Y")))
+    Z = make_immutable(PauliOp(Pauli("Z")))
+    I = make_immutable(PauliOp(Pauli("I")))
 
-# Clifford+T, and some other common non-parameterized gates
-CX = make_immutable(CircuitOp(CXGate()))
-S = make_immutable(CircuitOp(SGate()))
-H = make_immutable(CircuitOp(HGate()))
-T = make_immutable(CircuitOp(TGate()))
-Swap = make_immutable(CircuitOp(SwapGate()))
-CZ = make_immutable(CircuitOp(CZGate()))
+    # Clifford+T, and some other common non-parameterized gates
+    CX = make_immutable(CircuitOp(CXGate()))
+    S = make_immutable(CircuitOp(SGate()))
+    H = make_immutable(CircuitOp(HGate()))
+    T = make_immutable(CircuitOp(TGate()))
+    Swap = make_immutable(CircuitOp(SwapGate()))
+    CZ = make_immutable(CircuitOp(CZGate()))
 
-# 1-Qubit states
-Zero = make_immutable(DictStateFn("0"))
-One = make_immutable(DictStateFn("1"))
-Plus = make_immutable(H.compose(Zero))
-Minus = make_immutable(H.compose(X).compose(Zero))
+    # 1-Qubit states
+    Zero = make_immutable(DictStateFn("0"))
+    One = make_immutable(DictStateFn("1"))
+    Plus = make_immutable(H.compose(Zero))
+    Minus = make_immutable(H.compose(X).compose(Zero))