encoding: avoid cyclic dependency around "parsers" in pure Python build
80f2b63dd83a brought "asciilower" and "import parsers" into
"encoding.py".
This works fine with "parsers" module in C implementation, but doesn't
with one in pure Python implementation, because the latter causes
cyclic dependency below and aborting execution:
util => i18n => encoding => parsers => util
This patch delays importing "parsers" module until it is really
needed, to avoid cyclic dependency around "parsers" in pure Python
build.
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null