Fix padwith in PadField #6137

Job	Run time
Type hints check	28s
Code health check	1m 27s
CodeQL analysis	3m 18s
pyca/cryptography test	21s
Build doc	5m 21s
ubuntu-latest 3.7 non_root -K scanner	3m 9s
ubuntu-latest 3.8 non_root -K scanner	3m 19s
ubuntu-latest 3.9 non_root -K scanner	3m 21s
ubuntu-latest 3.10 non_root -K scanner	3m 11s
ubuntu-latest 3.11 non_root -K scanner	3m 1s
ubuntu-latest 3.12 non_root -K scanner	3m 29s
ubuntu-latest 3.12 root -K scanner	4m 32s
ubuntu-latest pypy3.9 root -K scanner	4m 14s
ubuntu-latest libpcap 3.12 root -K scanner	5m 4s
macos-12 3.12 both -K scanner	7m 56s
ubuntu-latest 3.12 root -k scanner	3m 59s
ubuntu-latest pypy3.9 root -k scanner	3m 1s
macos-12 3.12 both -k scanner	4m 2s
	1h 3m 13s

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