BUG: `ScopingPolicy` members improperly typed

[TTsangSC]

Description

Somehow mypy isn't recognizing that the ScopingPolicy members are its own instances and are hence strings; maybe it has to do with the hacky backport we have for enum.StrEnum (line_profiler.line_profiler_utils._StrEnumBase).

Example script (scoping-policy-type-bug.py)

from line_profiler.line_profiler import LineProfiler
from line_profiler.scoping_policy import ScopingPolicy


class MyClass:
    pass


def main() -> None:
    prof = LineProfiler()
    prof.add_class(MyClass, scoping_policy=ScopingPolicy.NONE)

Output

$ mypy scoping-policy-enum-bug.py
scoping-policy-type-bug.py:11 error: argument "scoping_policy" to "add_class" of "LineProfiler" has incompatible type "auto"; expected "ScopingPolicy | str | ScopingPolicyDict | None"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

Mitigation

I noticed that in #419 we're circumventing this within the codebase with manual type-casting (cast(ScopingPolicy, ScopingPolicy.<attr>); see d1b4c23). In some sense this is alright, since end users are more likely to just use the string forms for LineProfiler.add_*(..., scoping_policy=<policy>)... but we should probably fix this more thoroughly since ScopingPolicy is public API, and having the imported enum class not work would be quite counterintuitive.

Wonder if something like

class _StrEnumBase(str, enum.Enum):
    ...


if TYPE_CHECKING:
    from enum import StrEnum as _StrEnumBase  # type: ignore[attr-defined]


class StringEnum(_StrEnumBase):
    ...

in line_profiler/line_profiler_utils.py would work... if not, we may have to use the enum.auto-subclassing workaround.