Fix locale ISO datetime parsing

[nkgotcode]

Fixes #1469.

This updates locale datetime parsing so composite locale directives are expanded through locale.nl_langinfo() before parsing. It also accepts the ISO-like %c form produced by locales such as en_DK.utf8 when the formatted value has an empty %Z timezone field.

The regression test covers a locale-provided datetime format of %Y-%m-%dT%T %Z with an input value that omits the timezone name.

Validation:

. .venv-oss1856/bin/activate && py.test tests/parse_datetime_test.py -q
# 54 passed, 14 warnings

. .venv-oss1856/bin/activate
TMPDIR=$(mktemp -d)
mkdir -p "$TMPDIR/calendar" "$TMPDIR/cache"
CONFIG="$TMPDIR/config"
printf '%s\n' '[calendars]' '  [[default]]' "    path = $TMPDIR/calendar" '    color = dark magenta' '' '[locale]' 'local_timezone = Europe/Paris' 'default_timezone = Europe/Paris' '' '[sqlite]' "path = $TMPDIR/cache/khal.db" > "$CONFIG"
env -u LC_ALL LANG=en_US.utf8 LC_TIME=en_DK.utf8 khal -c "$CONFIG" printformats
env -u LC_ALL LANG=en_US.utf8 LC_TIME=en_DK.utf8 khal -c "$CONFIG" at '2026-05-14T12:00:00'
# exit 0; no critical parse error