Cap the Mercurial development dependency on Python 2.7.
Cap the Mercurial development dependency on Python 2.7.

About an hour ago, Mercurial released version 6.2, which drops support
for Python 2.7. This broke CI, preventing the package from installing.

This is a trivial change that caps the version used on Python 2.7 to
the 6.1 series.

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