Use `typing` library, `typing as t`

[tony]

• Use typing as t to namespace typing conveniently
• Use TypedDict and Literal from typing (instead of typing_extensions) since we're on python 3.8

Extracted from #274

[codecov[bot]]

Codecov Report

Merging #276 (e02edc7) into master (f84ed88) will decrease coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #276      +/-   ##
==========================================
- Coverage   93.31%   93.28%   -0.04%     
==========================================
  Files          13       13              
  Lines        1017     1012       -5     
  Branches      172      172              
==========================================
- Hits          949      944       -5     
  Misses         49       49              
  Partials       19       19              

Impacted Files	Coverage Δ
src/unihan_etl/test.py	100.00% <ø> (ø)
src/unihan_etl/expansion.py	100.00% <100.00%> (ø)
src/unihan_etl/process.py	85.39% <100.00%> (-0.07%)	:arrow_down:
src/unihan_etl/types.py	100.00% <100.00%> (ø)
tests/test_expansion.py	98.52% <100.00%> (+0.01%)	:arrow_up:

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