Prepare release 8.0.15

BUG29034868: Fix wrong WL number in CHANGES.txt

BUG28673129: Client.close() method raises socket error on Py2.7

A second call to the close method of Client can raise a
sockect error. This seems to be a different behavios in the
sockect library on Python 2.7, This Patch adds a try catch
to avoid raising the error.

This patch also fixes some failing unittests on Pb2 machines.

BUG28443941: differences on pure and c-ext in cmd_change_user()

The invocation of the method cmd_change_user() using the c extension
does not return any status information while the pure python
implementation it does.

This patch adds the missing status results for the cmd_change_user(),
in addition this patchs renames the server_status atribute name to
status_flag from the status information returned from fetch_eof_status()
to be consistent with the name.

Update CPYINT branch

BUG27837814: Fix missing .pyc files in Python 3 commercial packages

Add pip installation instructions in the docs

BUG26847553: Fix AttributeError when using pure Python Protobuf

Using a value of type list when setting an attribute in a Message object
throws an AttributeError.

This patch fix this issue by testing the value type and using the .extend()
method for list type values.

This patch also adds the encoding of string values types to bytes when using
Python 3.

Update CPYINT revision

Update CPYINT revision