Fixes #1788

Description

LfMerge has been on Mercurial 3 for a long time, and we've had constant trouble with HTTPS certificates because our Mercurial version was too old to support SNI. Moving to Mercurial 6 (and Python 3) will fix that.

Screenshots

Demonstrate any UI / behavioral changes with screenshots or animations.

Checklist

• I have labeled my PR with: bug, feature, engineering, security fix or testing
• I have performed a self-review of my own code
• I have reviewed the title & description of this PR which I will use as the squashed PR commit message
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have enabled auto-merge (optional)

Testing

Testers, use the following instructions against our staging environment. Post your findings as a comment and include any meaningful screenshots, etc.

Describe how to verify your changes and provide any necessary test data.

• Do multiple Send/Receives (a clone, a pull, a push, and at least one merge where the project is edited in FLEx and in Language Forge at the same time), and make sure they all succeed.