Added type annotations to curtsiesfrontend/interpreter.py: BPythonFormatter.format()

[amuletcoin]

Finished adding type hints to curtsiesfrontend/interpreter.py's functions, specifically the BPythonFormatter class's format() method.

[codecov-commenter]

Codecov Report

Merging #961 (5df8754) into main (f076f32) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #961      +/-   ##
==========================================
- Coverage   68.76%   68.74%   -0.03%     
==========================================
  Files          63       63              
  Lines        9524     9524              
==========================================
- Hits         6549     6547       -2     
- Misses       2975     2977       +2     

Impacted Files	Coverage Δ
bpython/curtsiesfrontend/interpreter.py	100.00% <100.00%> (ø)
bpython/repl.py	73.79% <0.00%> (-0.28%)	⬇️
bpython/test/test_importcompletion.py	92.00% <0.00%> (ø)

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[sebastinas]

Sorry, miss clicked.

The black regression is easy to fix afterwards, but the mypy regressions needs to be fixed before this can go forward.