Clarify dependencies

In the Installation section in the README.md you mention a dependency urlparse, which on pypi.org does not exist and later in the README.md in an according section is not mentioned anymore. I find dependency managegemnt via pip-tools very convenient, so I prepared a Merge Request. Anyway, if for any reason, you do not want to introduce a requirements.in or even not a requirements.txt, of course it would be sufficient to correct the part in the README.md, where urlparse is mentioned. Let me know, if you prefer that and what dependency is needed or that it can be deleted and I will happily do this.