Cirq v1.6.0 release

[Pavol Juhas]

Dear Cirq Users,

We are pleased to announce the new minor release for Cirq version v1.6.0!
The release is available on PyPI.  Please install or upgrade using

  pip install "cirq==1.6.0"

This release brings new functions, bug fixes, and compatibility updates.
The cirq-rigetti package has been removed and is not included with cirq
anymore (please use the last version cirq-rigetti-1.5.0 for legacy code needs).

General code maintenance

Increased type-check coverage by adding type annotations to unit tests
(an ongoing effort).  Updated annotations syntax to confirm to PEP-585.
Activated ruff tool for fast code checking and linting.  Deleted the
outdated maintenance tool pr_monitor and eliminated the use of Docker
in tools and tests.

cirq-core

Added CXSWAP and CZSWAP gates.  Added transformers for manipulating
operation tags and for parameterizing tagged operations.  Added support
for parameterized circuits in the merge-single-qubit-gates transformer.
Added a new implementation of parallel XEB accepting circuits instead of
single gates.  Improved Randomized Benchmarking to reduce the size of
generated circuits.  Simplified decomposition of controlled gates with
global phase.  Added JSON serialization support for `NoiseModel` classes.
Extended the QASM import parser to understand more OpenQASM 2.0 gates.
Improved the output and speed of Quantum Shannon Decomposition.
Implemented support for adding tags to Cirq moments.
Enhanced `apply_unitary` and `unitary` to accept numpy matrices.

API change

Changed CircuitOperation to assume `use_repetition_ids=False` by default
(you can pass the explicit argument `use_repetition_ids=True` to restore
the previous behavior).

cirq-google

Added specifications for the new Quantum Virtual Machine (QVM) "willow_pink"
with updated demos in QVM notebooks.  Improved serialization of Cirq objects
for Quantum Engine (QE) for better performance and to support more Cirq types.
Added classes to express analog qubit frequency and coupling experiments at QE.
Deprecated hard-coded default gate times in `noise_properties_from_calibration`.

cirq-ionq

Added support for expressing IonQ Pauli-exponential gates.
Fixed a bug in the computation of inverse MSGate.

cirq-web

Improved the cirq-web package layout to avoid installing the unnecessary cirq_ts module.

A Huge Thank You

Thank you to all our contributors for this release:

ACE07-Sev, Akshita, Andreas Bengtsson, Bicheng Ying, Codrut Grosu, Danni Wang,
Dax Fohl, Devon Yates, Doug Strain, Kerry Zhou, Michael Hucka, Michael Qian,
Nour Yosri, Pavol Juhas, Radu Marginean, Renyi Chen, Revanth Gundala,
Saurav Maheshkar, Seneca Meeks, Su Ji Park, WingCode

For detailed release notes please see https://github.com/quantumlib/Cirq/releases/tag/v1.6.0