Bug#946561: hplip: Not installable with python3 >= 3.8

["Stefano F." <issues@deeperception.eu>]

Package: hplip
Severity: grave
Justification: renders package unusable

-- System Information:
Debian Release: bullseye/sid
  APT prefers testing
  APT policy: (900, 'testing'), (700, 'experimental'), (300, 'unstable')
Architecture: amd64 (x86_64)

Kernel: Linux 5.3.0-2-amd64 (SMP w/4 CPU cores)
Kernel taint flags: TAINT_OOT_MODULE, TAINT_UNSIGNED_MODULE
Locale: LANG=it_IT.UTF-8, LC_CTYPE=it_IT.UTF-8 (charmap=UTF-8),
LANGUAGE=it_IT.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages hplip depends on:
ii  adduser                3.118
ii  cups                   2.3.0-7
pn  hplip-data             <none>
ii  libc6                  2.29-6
ii  libcups2               2.3.0-7
ii  libdbus-1-3            1.12.16-2
ii  libhpmud0              3.19.11+dfsg0-1
ii  libpython3.7           3.7.5-2
ii  libsane                1.0.27-3.2+b1
pn  libsane-hpaio          <none>
ii  lsb-base               11.1.0
ii  printer-driver-hpcups  3.19.11+dfsg0-1
ii  python3                3.8.0-2
ii  python3-dbus           1.2.14-1
ii  python3-gi             3.34.0-3
pn  python3-pexpect        <none>
pn  python3-pil            <none>
pn  python3-reportlab      <none>
ii  wget                   1.20.3-1+b2
ii  xz-utils               5.2.4-1+b1

Versions of packages hplip recommends:
ii  avahi-daemon                  0.7-4+b1
ii  policykit-1                   0.105-26
ii  printer-driver-postscript-hp  3.19.11+dfsg0-1
ii  sane-utils                    1.0.27-3.2+b1

Versions of packages hplip suggests:
pn  hplip-doc              <none>
pn  hplip-gui              <none>
pn  python3-notify2        <none>
ii  system-config-printer  1.5.12-1