overly strict dependency on proto-plus

[tswast]

We noticed in building an internal tool that the Spanner dependency is much more strict than in other packages, such as BigQuery.

python-spanner/setup.py

Line 36 in 1b0ce1d

"proto-plus==1.14.2",

Since proto-plus is a core dependency of all client libraries, it is often bundled with an environment (e.g. in AI Platform Notebooks). The strict dependency makes it much harder to upgrade the google-cloud-spanner library in these environments.

[tswast]

Note: If the worry is incompatibilities with older versions, we do have a way of testing this. In BigQuery we do this with constraints files.

https://github.com/googleapis/python-bigquery/blob/7447f05f6a92bc4c047dff9d0377598b7af15f18/noxfile.py#L122

There is a PR in the works to use constraints files across all generated clients. googleapis/synthtool#869

[larkee]

The strictness of the dependency came from an issue during the migration to v2.0 when I needed to pin to v1.10-dev2. It seems when I upgraded to 1.11.0 I kept the == rather than using >=. I've written a PR to update the dependency 👍