add numpy again to the distribution include path

Commit a8b0463e turned out to be a bad
idea: unlike on Debian, on many platforms the numpy headers are not
available in the default include path.
6 jobs for v1.3.0a1 in 2 minutes and 44 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed build package #36829

00:01:36

passed check for whitespace #36830

00:00:21

 
  Test
failed build documentation #36832

00:01:07

failed check for broken links in doc #36834
allowed to fail

00:01:03

passed check for dependencies installed #36831

00:00:31

passed run tests #36833

00:00:58

82.0%
 
Name Stage Failure
failed
check for broken links in doc Test

writing output... [100%] tutorial/tutorial6


build finished with problems, 1 warning.
make: *** [linkcheck] Error 1
Makefile:114: recipe for target 'linkcheck' failed
make: Leaving directory '/builds/kwant/kwant/doc'
ERROR: Job failed: exit code 1
failed
build documentation Test
loading pickled environment... not yet created

Warning, treated as error:
WARNING: latex_use_modindex is deprecated. Use latex_domain_indices instead.

make: *** [html] Error 1
Makefile:67: recipe for target 'html' failed
make: Leaving directory '/builds/kwant/kwant/doc'
ERROR: Job failed: exit code 1