CI fails for py35

[elacuesta]

https://github.com/elacuesta/scrapy-itemadapter/runs/607161233

It seems like it cann't find the README.md file, which is being opened in setup.py 🤷

I ran a debug job to list the contents of the current directory and it's there, perhaps setup.py is being executed from a different directory.

[elacuesta]

Solved in d9773fb by including a manifest file