Changelog

New Features

Adds a method prettyprint() to QuadraticProgram to generate a pretty-printed string of the object.
Adds informative text formats to str and repr functions of the following objects. The formats are IDE friendly, i.e., the text is one line.
```
 LinearConstraint
 LinearExpression
 QuadraticConstraint
 QuadraticExpression
 QuadraticObjective
 Variable
```
Adds a method prettyprint() to OptimizationResult to display the result in a multi-line text format.

If users set an empty variable name "" with binary_var(), integer_var(), and continuous_var(), they set the default variable name (e.g., x0) while they used to set the empty name as variable name.
Added support for running with Python 3.10. At the the time of the release, Cplex didn’t have a python 3.10 version and Docplex failed inside docplex.mp.model.Model.binary_var_list().
Updates the text format of str and repr of the following objects so that the output is one line.

QuadraticProgram
OptimizationResult
The previously deprecated BaseBackend class has been removed. It was originally deprecated in the Qiskit Terra 0.18.0 release.
Enable installation of CPLEX for Python 3.10.
Support for running with Python 3.6 has been removed. To run Optimization you need a minimum Python version of 3.7.

Fixed an issue that from_ising() raises an error when Pauli I is given.
Fixed an issue that to_ising() returns a wrong operator when there is no variable in an input problem.

Shows a warning message if non-printable strings are set to QuadraticProgram as problem name, variable name, or constraint name.
Updated the documentation of SUCCESS of OptimizationResultStatus. SUCCESS means the obtained solution is feasible, but not always optimal because some algorithms do not guarantee the optimality.
Reword the documentation of all methods and the multi-line text format of OptimizationResult as follows because some algorithms do not guarantee the optimality.
“optimal function value” → “objective function value”
“optimal value” → “variable values”