Tighten up type of Deploy._process_exports

[rswarbrick]

Firstly, the stricter return type is needed because the CovMerge constructor updates self.exports (which is generated with that function). We also know the key and value types, so we may as well give them as well.

The rest of the change is more complicated and a little ugly (we're confusing Pyright on purpose!). The trick is to pass the value that we wrote to self.exports from the hjson file as an argument to _process_exports. Doing it like this avoids Pyright trying to check that the field is both a list of dictionaries and a merged dictionary.

[AlexJones0]

This looks reasonable - below I've suggested an alternative pattern which may feel like a bit less of a dance around the type checker, but I'll leave it up to you if you prefer this or your existing solution:

def __init__(self, ...):
    self.exports: list[dict[str, str]] = []
    self.merged_exports: dict[str, str] = {}
    # ... self._define_attrs() then set_attrs, check_attrs, subst_vars, etc. ...
    self.merged_export = self._process_exports(self.exports)
    self.exports.clear()  # if we're being really paranoid about memory usage (likely unnecessary)
    # ...

def get_job_spec(self) -> "JobSpec":
    # ...
        exports=self.merged_exports,
    # ...

[AlexJones0]

Minor nit:

Suggested change

	def _process_exports(self, exports_list: list[dict[str, str]]) -> dict[str, str]:
	def _process_exports(self, exports_list: Iterable[Mapping[str, str]]) -> dict[str, str]: