Added doc example to convert date.max to infinity

See issue #163.
Untrack the connection before closing to avoid possible double-free

From Gangadharan S.A. Fixes issue #166.
Fixed doc index entry for infinite dates
Fixed manifest to avoid spurious messages during install

It was a while we were getting::

  no previously-included directories found matching 'doc/src/_build'

at install time. Also avoid a couple of Sphinx build files from sdist
package.
Bump up version for release 2.5.1