Some distros don't come with python2 and only have python3 available.
This causes the "'import perf' in python" self test to fail.

This change adds python3 to the list of possible python versions
that are autodetected but maintains the priorities for
'python2' and 'python' detection. Python3 has the lowest priority.

Committer notes:

On a fedora system without python2 packages the 'perf test python'
continues to work:

  # python2
  bash: python2: command not found...
  Similar command is: 'python'
  # rpm -qa | grep python2
  #

That "Similar command" gives the clue:

  # rpm -qf /usr/bin/python
  python-unversioned-command-3.8.5-5.fc32.noarch
  # rpm -ql python-unversioned-command
  /usr/bin/python
  /usr/share/man/man1/python.1.gz
  #

With it in place the 'python' binary is found and perf builds the python
binding using python3:

  # perf test -v python
  19: 'import perf' in python                                         :
  --- start ---
  test child forked, pid 379988
  python usage test: "echo "import sys ; sys.path.append('/tmp/build/perf/python'); import perf" | '/usr/bin/python' "
  test child finished with 0
  ---- end ----
  'import perf' in python: Ok
  #

Looking at that path:

  # ls -la /tmp/build/perf/python
  total 1864
  drwxrwxr-x.  2 acme acme      60 Oct 13 16:20 .
  drwxrwxr-x. 18 acme acme    4420 Oct 13 16:28 ..
  -rwxrwxr-x.  1 acme acme 1907216 Oct 13 16:28 perf.cpython-38-x86_64-linux-gnu.so
  #

And:

  # ldd ~/bin/perf | grep python
  	libpython3.8.so.1.0 => /lib64/libpython3.8.so.1.0 (0x00007f5471187000)
  #

As soon as we remove it:

  # rpm -e python-unversioned-command-3.8.5-5.fc32.noarch
  # hash -r
  # python
  bash: python: command not found...
  Install package 'python-unversioned-command' to provide command 'python'? [N/y] n
  #

And rebuilding perf now doesn't find python in the system:

  make: Entering directory '/home/acme/git/perf/tools/perf'
    BUILD:   Doing 'make -j24' parallel build
  <SNIP>
  Makefile.config:786: No python interpreter was found: disables Python support - please install python-devel/python-dev
  <SNIP>

After this patch:

  $ rpm -qi python-unversioned-command
  package python-unversioned-command is not installed
  $
  $ python
  bash: python: command not found...
  Install package 'python-unversioned-command' to provide command 'python'? [N/y] ^C
  $
  $ m
  make: Entering directory '/home/acme/git/perf/tools/perf'
    BUILD:   Doing 'make -j24' parallel build
  <SNIP>
    CC       /tmp/build/perf/tests/attr.o
    CC       /tmp/build/perf/tests/python-use.o
    DESCEND  plugins
    GEN      /tmp/build/perf/python/perf.so
    INSTALL  trace_plugins
    LD       /tmp/build/perf/tests/perf-in.o
    LD       /tmp/build/perf/perf-in.o
    LINK     /tmp/build/perf/perf
  <SNIP>
  make: Leaving directory '/home/acme/git/perf/tools/perf'
  19: 'import perf' in python                                         : Ok
  $ ldd ~/bin/perf | grep python
  	libpython3.8.so.1.0 => /lib64/libpython3.8.so.1.0 (0x00007f2c8c708000)
  $ ls -la /tmp/build/perf/python
  total 1864
  drwxrwxr-x.  2 acme acme      60 Oct 13 16:20 .
  drwxrwxr-x. 18 acme acme    4420 Oct 13 16:31 ..
  -rwxrwxr-x.  1 acme acme 1907216 Oct 13 16:31 perf.cpython-38-x86_64-linux-gnu.so
  $
Signed-off-by: NJames Clark <james.clark@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
LPU-Reference: 20201005080645.6588-1-james.clark@arm.com
Signed-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>