Drop dependency on rustls-pemfile #766

Job	Run time
msrv	1m 6s
Check for documentation errors	26s
Clippy	54s
features	1m 40s
Format	7s
Check semver compatibility	1m 5s
Build+test (macos-latest, stable)	3m 9s
Build+test (beta, ubuntu-latest)	3m 13s
Build+test (nightly, ubuntu-latest)	3m 7s
Build+test (stable, ubuntu-latest)	3m 11s
Build+test (windows-latest, stable)	6m 3s
	24m 1s

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