drop Python 3.8 support

[njzjz]

The upstream package has a bug in Python 3.8 (materialsproject/api#809), so we decide to drop Python 3.8 support.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.07 ⚠️

Comparison is base (0b8b175) 47.86% compared to head (7e4f5a4) 47.79%.

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #1264      +/-   ##
==========================================
- Coverage   47.86%   47.79%   -0.07%     
==========================================
  Files          82       82              
  Lines       14558    14586      +28     
==========================================
+ Hits         6968     6972       +4     
- Misses       7590     7614      +24     

Impacted Files	Coverage Δ
dpgen/generator/run.py	66.56% <ø> (-0.02%)	⬇️
dpgen/arginfo.py	92.59% <100.00%> (ø)
dpgen/data/arginfo.py	96.29% <100.00%> (-0.05%)	⬇️
dpgen/dispatcher/Dispatcher.py	83.78% <100.00%> (-0.43%)	⬇️
dpgen/generator/arginfo.py	100.00% <100.00%> (ø)
dpgen/simplify/arginfo.py	100.00% <100.00%> (ø)
dpgen/simplify/simplify.py	22.18% <100.00%> (ø)
dpgen/util.py	94.62% <100.00%> (+0.05%)	⬆️

... and 20 files with indirect coverage changes

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