Fix static typing

[cffls]

Fixed the following files:

pycardano/certificate.py
pycardano/coinselection.py
pycardano/key.py
pycardano/metadata.py
pycardano/plutus.py
pycardano/serialization.py
pycardano/transaction.py
pycardano/txbuilder.py
pycardano/utils.py
pycardano/witness.py

[codecov-commenter]

Codecov Report

Merging #139 (6d98a01) into main (5e37db1) will increase coverage by 0.01%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main     #139      +/-   ##
==========================================
+ Coverage   86.43%   86.44%   +0.01%     
==========================================
  Files          26       26              
  Lines        2734     2752      +18     
  Branches      649      651       +2     
==========================================
+ Hits         2363     2379      +16     
- Misses        279      281       +2     
  Partials       92       92              

Impacted Files	Coverage Δ
pycardano/coinselection.py	91.73% <50.00%> (ø)
pycardano/txbuilder.py	91.53% <75.67%> (-0.32%)	⬇️
pycardano/certificate.py	92.59% <100.00%> (ø)
pycardano/cip/cip8.py	100.00% <100.00%> (ø)
pycardano/key.py	87.67% <100.00%> (+0.17%)	⬆️
pycardano/metadata.py	94.59% <100.00%> (ø)
pycardano/plutus.py	92.57% <100.00%> (+0.07%)	⬆️
pycardano/serialization.py	88.93% <100.00%> (+0.09%)	⬆️
pycardano/transaction.py	94.37% <100.00%> (+0.07%)	⬆️
pycardano/utils.py	94.82% <100.00%> (ø)
... and 1 more

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