MRG: MAINT/0.1 --> fixes to build old docs without errors

[sappelhoff]

NOTE: This goes into maint/0.1 ... NOT master :-)

I am still trying to get simple versioned docs (see #364)

Doing that, I am going through our old releases and generating the docs as they were then.

Small fixes --> note: this is NOT about fixing past mistakes. Only to get the docs to work without warnings+errors.

I am going to do this for 0.2 and 0.3 as well and then hopefully make a PR to the gh-pages branch with the old docs.

Edit: To recreate the old environment, I started with a fresh conda env and then used the old .circle/config.yml file to install the adequate versions. When only master was specified, I compared release dates to figure out what version master was at that time.

Merge checklist

Maintainer, please confirm the following before merging:

• All comments resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• PR description includes phrase "closes <#issue-number>"
• Commit history does not contain any merge commits

[jasmainak]

Thanks @sappelhoff !