Remove Python 3.8 compat in .macro.extrapolate()

[khaeru]

pandas-stubs version 2.3.0 was released 2025-07-02 (lagging about a month behind pandas itself). This caused mypy errors here:

message_ix/macro.py:283: error: No overload variant of "apply" of "DataFrameGroupBy" matches argument types "Callable[[DataFrame], float]", "dict[str, bool]"  [call-overload]
message_ix/macro.py:283: note: Possible overload variants:
message_ix/macro.py:283: note:     def [P`17067] apply(self, DFCallable1[P], /, *args: P.args, **kwargs: P.kwargs) -> Series[Any]
message_ix/macro.py:283: note:     def [P`17068] apply(self, DFCallable2[P], /, *args: P.args, **kwargs: P.kwargs) -> DataFrame
message_ix/macro.py:283: note:     def [P`17069] apply(self, DFCallable3[P], /, *args: P.args, **kwargs: P.kwargs) -> DataFrame

The offending code was for compatibility with Python 3.8 and earlier, or at least with versions of pandas that would be installed for Python 3.8. Since this Python version is no longer supported, this PR drops the work-around, which makes the error disappear.

How to review

Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation. N/A, type checking only
• Update release notes. Ditto.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.8%. Comparing base (99514bf) to head (08606f2).
⚠️ Report is 82 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #960     +/-   ##
=======================================
- Coverage   96.0%   95.8%   -0.3%     
=======================================
  Files         53      53             
  Lines       5028    5026      -2     
=======================================
- Hits        4831    4818     -13     
- Misses       197     208     +11     

Files with missing lines	Coverage Δ
message_ix/macro.py	96.7% <ø> (-0.1%)	⬇️

... and 1 file with indirect coverage changes

[khaeru]

The failing codecov/project check appears to be spurious.