Update README.md
[REM] report: remove openerp/report/pyPdf

Distributing our version of pyPdf is not required anymore since we
introduced the new reporting tool.

In order to keep reports working, the standard python library
`python-pypdf` has to be installed instead (through pip or your
distribution's package manager)
[FIX] models: make field inheritance work when source field is defined in old api