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.
9 jobs for readme-drop-note-about-debian-jessie in 2 minutes and 15 seconds (queued for 3 seconds)
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