Remove Python requirement from ReadMe

[tcdejong]

screeps-server-mockup is removed in #113, which relied on screeps and in turn on node-gyp, which required Python 2.
No other packages depend on Python 2. Since #113 is merged, Python 2 is no longer required for this repository.

This PR updates the repository's readme accordingly.

[pyrodogg]

I merged the PR to remove screeps-server-mockup as more of a stopgap measure. I found that it really was failing to compile because it hadn't been maintained. It's dependencies have been updated and it now works with the latest screeps version.

I'd like more discussion on #117 regarding how much of a test suite should be in the starter package before continuing to remove and clean up integration testing.

In either case, it looks like node-gyp has supported Python 3 since 3.5.

[tcdejong]

I'd still argue this should be merged until Python is required again, and then re-add it with correct versions mentioned. I'll go over the discussion as I see more has been said since I last checked.