The version of Unicode that the Unicode parts of
str methods are based on.
New versions of Unicode are released regularly and subsequently all methods
in the standard library depending on Unicode are updated. Therefore the
behavior of some
str methods and the value of this constant
changes over time. This is not considered to be a breaking change.
The version numbering scheme is explained in Unicode 11.0 or later, Section 3.1 Versions of the Unicode Standard.