Updates to Python 2.7 What's New preamble

- refers to release PEP for lifecycle information
- refers to Python Package Index for migration tools
- covers enhancements added in maintenance releases

Closes issue #21569
allow the keyword else immediately after (no space) an integer (closes #21642)
document the compress_level argument to tarfile.open (closes #21404)

Patch by Katherine Busch.
specify that getuid() returns the real uid (closes #10503)

Patch by εσχατοκυριος.
don't remove self from example code in the HTML output (closes #13223)

Patch by Víctor Terrón.
document TokenError and unclosed expression behavior (closes #12063)

Patch by Amandine Lee.
backed out 86ba41b7bb46 (#18910) for test breakage
make sure the builtin help function doesn't fail when sys.stdin is not a valid file (closes #11709)

Original patch by Amaury Forgeot d'Arc with a test by bdettmer.