Strings that could be interpreted as sexagesimal should always be emitted quoted

Not doing so causes parsers that follow the yaml 1.1 spec (crazy though it might be), to load certain unquoted mac addresses as integers.

IMO https://github.com/ruby/psych/issues/265 wasn't actually a bug. It was following the 1.1 spec. (To make things more complicated, sexagesimal was dropped from yaml in the 1.2 spec)

Ruby 2.5 and 2.6 emit mac addresses, (but not shorter strings it thinks are more likely to be sexagesimal), unquoted.

2.6.0 :002 > '52:54:00:44:49:56'.to_yaml
 => "--- 52:54:00:44:49:56\n...\n" 
2.6.0 :003 > '44:49:56'.to_yaml
 => "--- '44:49:56'\n"

ruby < 2.5 then parse them as integers.

2.4.5 :006 >   YAML.load('macaddress: 52:54:00:44:49:56')
 => {"macaddress"=>12465480}

Here is an actual example of where the change in handling of sexagesimal has been a problem.

Start by tracing the to_yaml emission path for strings and compare it with the Ruby 2.5 and 2.6 examples in the issue. Check how YAML 1.1 parsers interpret the emitted values, then verify that sexagesimal-like strings such as MAC addresses are quoted without changing other string output.