pip_requirements: ease the dependency on specific version of modules.

Since it's not necessary to explicitly tie XinUI with modules such as
ryu==3.6, etc., I think it's okat to let users meet requirements if
they have newer modules installed.

Signed-off-by: Wei-Li Tang <alextwl@xinguard.com>
Merge pull request #1 from alextwl/master

pip_requirements: ease the dependency on specific version of modules.