Make 5.1 branch compatible with Python 3.10

[rochoa]

The Python 3.10 version finally removed the deprecated aliases for ABC collections classes, see https://docs.python.org/3/whatsnew/3.10.html#removed.

Migration to collections.abc.MutableMapping happened at https://github.com/tornadoweb/tornado/commit/6dceb64ed27c1d48af22142f2ebae946f0e85e95 but never made it to the 5.1 branch.

This makes possible to run Tornado 5.1.x in Python 3.10.