usertools: fix pmdinfo with python 3 and pyelftools>=0.24

usertools: fix pmdinfo with python 3 and pyelftools>=0.24

Running dpdk-pmdinfo.py on Ubuntu 18.04 (bionic) with python 3 and
pyelftools installed produces no output but no error is reported
neither:

  ~$ python3 usertools/dpdk-pmdinfo.py -r build/app/testpmd
  ~$ echo $?
  0

While with python 2, it works:

  ~# python2 usertools/dpdk-pmdinfo.py -r build/app/testpmd
  {"pci_ids": [], "name": "dpio"}
  {"pci_ids": [], "name": "dpbp"}
  {"pci_ids": [], "name": "dpaa2_qdma"}
  .....

On Ubuntu 18.04, pyelftools is version 0.24. The change log of
pyelftools v0.24 says:

 - Symbol/section names are strings internally now, not bytestrings
   (this may affect API usage in Python 3) (#76).

We cannot guess which version of pyelftools is actually being used. The
elftools.__version__ symbol is not consistent with each distro's package
version. For example, on Ubuntu 16.04 (xenial), the .deb package version
is '0.23-2' but elftools.__version__ contains '0.25'. This is certainly
due to partial backports.

To have a more consistent behaviour of this script across all versions
of python, add the unicode_literals future import so that literal
strings are now always "unicode".

Add 2 utility functions to force a string into bytes or bytes into an
unicode string.

Force pyelftools return values to unicode strings (will do nothing with
recent version of pyelftools).

If elffile.get_section_by_name returns None with a unicode section name,
try with the same one encoded as bytes.

Also, replace all open() calls by io.open() which behaves like the
builtin open in python 3. The only non-binary opened file is
/usr/share/hwdata/pci.ids which is UTF-8 encoded text. Explicitly
specify that encoding.

Link: https://github.com/eliben/pyelftools/blob/v0.24/CHANGES#L7
Link: https://github.com/eliben/pyelftools/commit/108eaea9e75a8b5a
Fixes: 54ca545dce4b ("make python scripts python2/3 compliant")
Cc: stable@dpdk.org
Signed-off-by: Robin Jarry <robin.jarry@6wind.com>
Reviewed-by: Olivier Matz <olivier.matz@6wind.com>