test_requirements: Avoid using pip as library
test_requirements: Avoid using pip as library

Because importing "pip" in Python scripts is not supported usage of
"pip", this patch fixes to use "pkg_resources" + "urllib" instead.

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

fujita/ryu,fujita/ryu,fujita/ryu,lagopus/ryu-lagopus-ext,lagopus/ryu-lagopus-ext,osrg/ryu,fujita/ryu,osrg/ryu,iwaseyusuke/ryu,iwaseyusuke/ryu,iwaseyusuke/ryu,lagopus/ryu-lagopus-ext,iwaseyusuke/ryu,fujita/ryu,osrg/ryu,lagopus/ryu-lagopus-ext,iwaseyusuke/ryu,osrg/ryu,osrg/ryu,lagopus/ryu-lagopus-ext