Consider replacing `mypy` with `ty`

[samcunliffe]

As we get more experience using it, we can form an opinion on replacing mypy with ty in both the template and recommendation pages.

From a first pass on a large(ish) codebase with some type hints (and probably some typing bugs) they do not behave identically out of the box:

mypy .
# ...
Found 243 errors in 25 files (checked 63 source files)

vs.

uvx ty check --output-format concise
# ...
Found 63 diagnostics

The latter was indeed very fast. The first thing ty does is warn us:

WARN ty is pre-release software and not ready for production use. Expect to encounter bugs, missing features, and fatal errors.

[paddyroddy]

Having played with this recently, I've noticed that some code bases use multiple of them, i.e. pandas-stubs tests on ty, pyrefly, mpy and pyright.

[paddyroddy]

I also think we should look into mypy_primer which is compatible with ty used by ty in CI.