use `to_lowercase` in `str downcase` #11172

Job	Run time
std-lib-and-python-virtualenv (ubuntu-20.04, py)	4m 16s
plugins (windows-latest)	5m 29s
std-lib-and-python-virtualenv (macos-latest, py)	7m 35s
fmt-clippy (windows-latest, default)	5m 50s
plugins (macos-latest)	1m 51s
tests (windows-latest, default)	11m 1s
std-lib-and-python-virtualenv (windows-latest, py)	11m 38s
fmt-clippy (windows-latest, extra)	6m 30s
plugins (ubuntu-20.04)	1m 15s
tests (macos-latest, default)	4m 48s
fmt-clippy (macos-latest, default)	2m 55s
tests (ubuntu-20.04, default)	4m 49s
fmt-clippy (macos-latest, extra)	3m 30s
fmt-clippy (ubuntu-20.04, default)	1m 32s
tests (ubuntu-20.04, dataframe)	11m 13s
fmt-clippy (ubuntu-20.04, dataframe)	3m 11s
tests (ubuntu-20.04, extra)	5m 14s
fmt-clippy (ubuntu-20.04, extra)	1m 37s
	1h 34m 14s

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