Provide consistent version of pyyaml no matter how you install faucet

[gizmoguy]

Faucet installed via pip and docker will always get the latest version of pyyaml (5.1.2 at time of writing).

Faucet installed via apt will get whatever the system version of pyyaml is (this could be quite old). Recently pyyaml in the 5.x branch have changed the behaviour of parsing colons which affects IPv6 addresses in faucet configuration.

I think we should modify requirements.txt to add an exact version match for pyyaml (pyyaml==5.1.2) and then ship our own debian packages for pyyaml so that faucet users get a consistent configuration experience no matter how they decide to install faucet.

[gizmoguy]

pyyaml 5.1 breaks a few things: yaml/pyyaml#265

I'm now thinking we should force ourselves to 3.13 pending pyyaml 5.2 which will fix a few things.