feat: Move vcs backend to asyncio

[dcramer]

The core goal here is to resolve some IO latency/blocking in vcs-server when git fetch takes a long period of time.

This upgrades us to Python 3.8.1 as well to make use of AsyncMock.

Note, pytest is locked at <5.4 right now, and is currently broken as of the 5.4.0 release.

[dcramer]

bors r+

[dcramer]

pytest bug that I discovered while doing this is here https://github.com/pytest-dev/pytest/issues/6909

[bors[bot]]

Build failed

• continuous-integration/travis-ci/push

[dcramer]

bors r+

[bors[bot]]

Build failed

• continuous-integration/travis-ci/push

[dcramer]

bors r+

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push