README: Drop note about Debian jessie, which is oldoldstable nowadays

As such, hopefully, it's not really used widely anymore. If so, this
note isn't really relevant.
Status Job ID Name Coverage
  Linting
passed #102072
linting:bandit

00:00:16

passed #102075
linting:mypy

00:00:16

passed #102074
linting:pyflakes

00:00:16

passed #102073
linting:pylint

00:00:24

 
  Test
passed #102076
tests:archlinux

00:01:27

passed #102077
tests:debian

00:01:23

98.0%
failed #102078
allowed to fail
tests:debian_with_bubblewrap

00:01:45

passed #102079
tests:fedora

00:01:27

failed #102080
allowed to fail
tests:gentoo

00:01:48

 
Name Stage Failure
failed
tests:gentoo Test
  File "/usr/lib64/python3.6/subprocess.py", line 418, in run
output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['/usr/bin/bwrap', '--ro-bind', '/usr', '/usr', '--ro-bind', '/lib', '/lib', '--ro-bind', '/lib64', '/lib64', '--ro-bind', '/bin', '/bin', '--ro-bind', '/sbin', '/sbin', '--ro-bind', '/builds/jvoisin/mat2', '/builds/jvoisin/mat2', '--ro-bind', '/etc/ld.so.cache', '/etc/ld.so.cache', '--dev', '/dev', '--chdir', '/builds/jvoisin/mat2', '--unshare-all', '--new-session', '--ro-bind', '/tmp/tmpo09ozsvi/tests/data/dirty.jpg', '/tmp/tmpo09ozsvi/tests/data/dirty.jpg', '/usr/bin/exiftool', '-json', '/tmp/tmpo09ozsvi/./tests/data/dirty.jpg']' returned non-zero exit status 1.

----------------------------------------------------------------------
Ran 129 tests in 94.376s

FAILED (errors=2)
ERROR: Job failed: exit code 1
failed
tests:debian_with_bubblewrap Test
  File "/usr/lib/python3.7/subprocess.py", line 487, in run
output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['/usr/bin/bwrap', '--ro-bind', '/usr', '/usr', '--ro-bind', '/lib', '/lib', '--ro-bind', '/lib64', '/lib64', '--ro-bind', '/bin', '/bin', '--ro-bind', '/sbin', '/sbin', '--ro-bind', '/builds/jvoisin/mat2', '/builds/jvoisin/mat2', '--ro-bind', '/etc/ld.so.cache', '/etc/ld.so.cache', '--dev', '/dev', '--chdir', '/builds/jvoisin/mat2', '--unshare-all', '--new-session', '--ro-bind', '/tmp/tmp6utrvi7t/tests/data/dirty.jpg', '/tmp/tmp6utrvi7t/tests/data/dirty.jpg', '/usr/bin/exiftool', '-json', '/tmp/tmp6utrvi7t/./tests/data/dirty.jpg']' returned non-zero exit status 1.

----------------------------------------------------------------------
Ran 129 tests in 92.270s

FAILED (errors=2)
ERROR: Job failed: exit code 1