Fix pytket.zx dependency issue

[CalMacCQ]

Description

There was an issue with the tensor_from_mixed_diagram function in the pytket.zx module. This was due to a new release of the quimb package version 1.7.0.

The 1.7.0 release dropped the opt_einsum dependency casuing the error. It seems there is now equivalent functionality in the cotengra package under the name of greedy instead of random-greedy.

Please summarise the changes.

Related issues

See the build failure in CQCL/pytket-docs#279

Checklist

• I have performed a self-review of my code.
• I have commented hard-to-understand parts of my code.
• I have made corresponding changes to the public API documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the changelog with any user-facing changes.