Bug#916195: python3-qwt: make optional the python3-pyqt4 dependency

[Marc Josep Rosanes Siscart]

Package: python3-qwt
Version: 0.5.5-1
Severity: important

Dear Maintainer,

When installing python3-qwt from Debian stretch (using apt-get install), 
python3-pyqt4 is also installed, even if python3-pyqt5 is available.

After testing python3-qwt with python3-pyqt5 and a dummy/mock package of python3-pyqt4 
(allowing to bypass the dependency on it), the impression is that python3-qwt is working well with python3-pyqt5.

The dependency on python3-pyqt4 should be made optional. 
If python3-pyqt5 is already installed in the system, python3-pyqt4 should not be installed.


-- System Information:
Debian Release: buster/sid
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 4.9.0-5-amd64 (SMP w/4 CPU cores)
Locale: LANG=C, LC_CTYPE=C (charmap=locale: Cannot set LC_ALL to default locale: No such file or directory
ANSI_X3.4-1968), LANGUAGE=en_US (charmap=locale: Cannot set LC_ALL to default locale: No such file or directory
ANSI_X3.4-1968)
Shell: /bin/sh linked to /bin/bash
Init: systemd (via /run/systemd/system)

Versions of packages python3-qwt depends on:
ii  python3        3.5.3-1
ii  python3-numpy  1:1.12.1-3
ii  python3-pyqt4  4.11.4+dfsg-2+b1

python3-qwt recommends no packages.

python3-qwt suggests no packages.

-- debconf information:
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
	LANGUAGE = "en_US",
	LC_ALL = (unset),
	LC_MEASUREMENT = "ca_ES.UTF-8",
	LC_MONETARY = "ca_ES.UTF-8",
	LC_COLLATE = "ca_ES.UTF-8",
	LANG = "C"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
locale: Cannot set LC_ALL to default locale: No such file or directory