gnuradio 3.8: Filter design tool does not open

[Dennis Real]

Steps to reproduce:

1. Open gnuradio-companion
2. Select Menu -> Tools -> Filter Design Tool
3. Nothing happens

OR

1. Run command "gr_filter_design"
2. Exits with fftpack failure

Workaround:

In file /usr/lib/python3/dist-packages/gnuradio/filter/filter_design.py change line
   from numpy.fft import fftpack as fft_detail
to
   import numpy.fft as fft_detail

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: gnuradio 3.8.1.0~rc1-2build2 [modified: usr/lib/python3/dist-packages/gnuradio/filter/filter_design.py]
ProcVersionSignature: Ubuntu 5.4.0-31.35-generic 5.4.34
Uname: Linux 5.4.0-31-generic x86_64
ApportVersion: 2.20.11-0ubuntu27
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: XFCE
Date: Fri May 22 18:36:59 2020
InstallationDate: Installed on 2016-04-30 (1483 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
SourcePackage: gnuradio
UpgradeStatus: Upgraded to focal on 2020-05-17 (5 days ago)
modified.conffile..etc.default.apport:
 # set this to 0 to disable apport, or to 1 to enable it
 # you can temporarily override this with
 # sudo service apport start force_start=1
 enabled=0
mtime.conffile..etc.default.apport: 2017-06-27T22:18:31.198313