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.