This introduces a non-compatible change: the next version will have to
be 10.x.x.

Not only we are adding a new dependency but also 'regex' requires a
compiler to be built.

Use a home-made regex module to replace Python's re. This home-made
delegates on the third-party regex module the real work: the only
purpose is to serve as a thin layer between client code and the real
regex engine.

Supports only compile and escape to promote the use the regex/pattern
objects instead of the global functions search/match/find/sub/...

Pexpect checks explicitly for Python's regexs rejecting Barnett's one.
The adapter closes the gap between this two and allow to use Barnett's
regex with pexpect.