Fix development dependencies for Python 2.6.

Review Request #9527 — Created Jan. 24, 2018 and submitted — Latest diff uploaded

Information

RBTools
release-0.7.x
adc76b4...

Reviewers

For development, RBTools depends on hgsubversion, which depends on any
version of mercurial. However, the latest version of mercurial does
not work on Python 2.6.

This change explicitly lists mercurial as a dependency and adds
version specifiers to cap the version on Python 2.6.

Tested installation on Python 2.6 and 2.7.