Move Mercurial dependency to pip.txt

[stsewd]

While replicating #3066 I was getting an error because of mercurial missing, a time ago I do this PR #3358 that add mercurial to the tests dependencies, but it's also required for the main project.

[berkerpeksag]

Normally installation of the Mercurial requirement is handled by the system package manager: https://github.com/rtfd/readthedocs-docker-images/blob/454e82e586b04340084055c2be5458dbe7a625f3/Dockerfile#L17

So adding it to the base requirement file may cause a problem on production. Also, looking at https://pypi.org/project/Mercurial/#files I think installing it via pip may require a compiler on some platforms (because it's not a pure Python project)

I'd suggest adding a requirements/dev.txt file or updating the installation document.

[humitos]

I think @berkerpeksag is right.

Because of the points mentioned, I think it's better to update the install docs to mention that mercurial is needed to have some extra features.

It's not an strict requirement (in case you want to contribute) --but if you want to work on something that depends on mercurial you will need to install it. So, adding this to the docs it's the best from my point of view.

[stsewd]

@humitos I thinking on improve the installation docs with this notes #3614 (comment), probably I will include the mercurial dependency there. Closing this for now.