Fix docs build

[carver]

What was wrong?

Docs have been failing for a while, RTD was configured assuming a requirements-docs.txt was present.

How can it be fixed?

There is no obvious way to get RTD to install a project "extra" for docs, so a hack known to work is to add a requirements-docs.txt with only one line:

web3[docs]

It would maybe be better to do .[docs] but I haven't confirmed that it works on RTD.