Fix: Perspective 1.7 is unsupported.

[cythrawll]

Description

Seeing some issues with the expression editor in perspective 1.7.

Proposed Changes

• For now keep on 1.6, until we can properly support 1.7
• https://github.com/finos/perspective/pull/1971

License

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.