DEV: Unify code between PdfReader and PdfWriter #4470

Job	Run time
Build & verify package	17s
pytest on windows	4m 54s
Check code style issues	35s
pytest on 3.7 (crypto-lib: cryptography)	3m 26s
pytest on 3.8 (crypto-lib: cryptography)	3m 42s
pytest on 3.9 (crypto-lib: cryptography)	3m 45s
pytest on 3.10 (crypto-lib: cryptography)	3m 37s
pytest on 3.11 (crypto-lib: cryptography)	3m 24s
pytest on 3.12 (crypto-lib: cryptography)	4m 46s
pytest on 3.7 (crypto-lib: pycryptodome)	3m 39s
pytest on 3.7 (crypto-lib: none)	3m 48s
Combine & check coverage.	16s
	36m 9s

Provide feedback