chore: drop python 3.8

[pfackeldey]

I hope I haven't forgotten any places?

[martindurant]

Should add 3.13 at the same time

[martindurant]

(need the same at dask-histogram too, but all CI is red right now)

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.98%. Comparing base (8cb8994) to head (12049c8).
Report is 213 commits behind head on main.

Files with missing lines	Patch %	Lines
src/dask_awkward/lib/io/io.py	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #569      +/-   ##
==========================================
- Coverage   93.06%   91.98%   -1.09%     
==========================================
  Files          23       22       -1     
  Lines        3290     3542     +252     
==========================================
+ Hits         3062     3258     +196     
- Misses        228      284      +56     

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[lgray]

conda tests will fail until dask histogram is updated in conda.

[lgray]

ok - looks like we're good to go. Need to check something over in uproot though.

[lgray]

Nope - right - uproot is OK because the tree reduction for the reports is here. Very good.

[pfackeldey]

any objections about merging this PR @martindurant and @lgray ?