Added the lstrip, strip, and rstrip interfaces to the String class (v2.8.5)

[simon-ritchie]

Since v2.8.5, the lstrip, strip, and rstrip interfaces have been added to the String class.

These interfaces are similar to the Python str built-in.

import apysc as ap

string: ap.String = ap.String("aabbccaa")
string = string.strip(string="a")
ap.assert_equal(string, "bbcc")

For more details, please see the following documents:

• String class lstrip interface
• String class strip interface
• String class rstrip interface