Fix mypy version check

[KotlinIsland]

Change Summary

Basically the mypy plugin is broken with any dev version of mypy, I changed how the version number is parsed.

>mypy --version 
mypy 0.940+dev.04cac4b5d911c4f9529e6ce86a27b44f28846f5d.dirty

>mypy test.py       
pyproject.toml:1: error: Error importing plugin "pydantic.mypy": could not convert string to float: 
Found 1 error in 1 file (errors prevented further checking)

Also mypy will switch to a version number like x.y.z which the current check will fail on. It will be better to future proof this check to prevent a sudden wave of reports of "could not convert string to float".

Related issue number

N/A

Checklist

• [N/A] Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[KotlinIsland]

please review

[samuelcolvin]

LGTM, please add a change description.

[KotlinIsland]

This is not optimal, it should ideally use the version string passed from mypy, not import from mypy.version.

[samuelcolvin]

Looking good, you'll need to rebase or merge master to get tests passing.

[samuelcolvin]

Please move this into a function so we can test some real life values.

[KotlinIsland]

Like that?

[samuelcolvin]

please update.

[samuelcolvin]

thanks so much.