Fix warning: unused function '__pyx_pw_5numpy_7ndarray_1__getbuffer__'

Originally fixed in 6cc61605, but a regression was later introduced in
519a19a9.

delete more redundant code when encountering constant bool conditions inside of generator expressions

raise overflow error in Py3.3+ when non-BMP Unicode characters are coerced into a short Py_UNICODE value

scripts with shebangs <-> executable + uniform use of #!/usr/bin/env python

fix analysis of entry to be a weakref (typo introduced in d96dfd)

Originally reported in Debian http://bugs.debian.org/cgi-bin/bugreport.cgi\?bug\=692313
Lead to crash with python-dbg:

  python-dbg: ../Modules/gcmodule.c:366: visit_decref: Assertion 'gc->gc.gc_refs != 0' failed

Minor cleanup of a couple of examples

- fixed wrong references (The file on the left (right) -> actual file name)
- divided an example snippet of three files into separate files

Fixed indentation of a code example

Fixed a code example which had been indented with 2 spaces.

changed file order in Sharing C Functions example

This order looks more logical, easier to read.