Make compatible with standard pip installation methods

[jrouly]

Because setup.py exists within a subfolder (src) and this package is not published on PyPi, it's impossible to install from git with package managers like poetry.

Moving setup.py to the top level and exposing the package instead of src makes this package more compatible.

[ChristianSch]

until this is merged, if at all, you can use this:

pip3 install 'openne @ git+https://github.com/thunlp/OpenNE.git@master#subdirectory=src'

[jrouly]

@helloxcq @zzy14 @eustomaqua @Freyr-Wings @skeletondyh Any chance of getting this merged? We've been maintaining an internal fork with this change for over a year now.