yaml parsing is wrong when key's value has leading colon

replication steps:

ruby -ryaml -e "a = YAML.load('servers: ::ffff:10.10.10.1'); puts a; puts a['servers']"
{"servers"=>:":ffff:10.10.10.1"}
:ffff:10.10.10.1

expected:

{"servers"=>"::ffff:10.10.10.1"}
::ffff:10.10.10.1

actual result:

{"servers"=>:":ffff:10.10.10.1"}
:ffff:10.10.10.1

Environment:

ruby --version
ruby 2.7.0p0

gem list yaml 
yaml (0.3.0)

Python yaml works as expected:

python3.9 -c 'import yaml; a=yaml.safe_load("servers: ::ffff:10.10.10.1"); print(a); print(a["servers"]);'
{'servers': '::ffff:10.10.10.1'}
::ffff:10.10.10.1

Reproduce the result with the Ruby YAML.load command shown in the issue, then trace how that input is handled by Psych and its libyaml wrapper. The fix is done when the leading double colon remains part of the scalar value and the displayed hash and value match the expected output.