Fix float resolver for '.' and '._'

A single dot matches the official YAML 1.1 int regex.
This was probably unintended. The regex now requires at least
a digit before or after the dot.

lib3/yaml/resolver.py

@@ -177,7 +177,7 @@ class Resolver(BaseResolver):
 Resolver.add_implicit_resolver(
         'tag:yaml.org,2002:float',
         re.compile(r'''^(?:[-+]?(?:[0-9][0-9_]*)\.[0-9_]*(?:[eE][-+][0-9]+)?
-                    |\.[0-9_]+(?:[eE][-+][0-9]+)?
+                    |\.[0-9][0-9_]*(?:[eE][-+][0-9]+)?
                     |[-+]?[0-9][0-9_]*(?::[0-5]?[0-9])+\.[0-9_]*
                     |[-+]?\.(?:inf|Inf|INF)
                     |\.(?:nan|NaN|NAN))$''', re.X),