Could not find a version that satisfies the requirement opencv-python

When I try to install the dependencies using

$% pip install -r requirements.txt

I get:

Collecting defusedxml==0.6.0
  Downloading defusedxml-0.6.0-py2.py3-none-any.whl (23 kB)
Collecting numpy==1.18.2
  Downloading numpy-1.18.2.zip (5.4 MB)
     |████████████████████████████████| 5.4 MB 3.6 MB/s 
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
Collecting odfpy==1.4.1
  Downloading odfpy-1.4.1.tar.gz (717 kB)
     |████████████████████████████████| 717 kB 2.8 MB/s 
ERROR: Could not find a version that satisfies the requirement opencv-python==4.2.0.34 (from -r requirements.txt (line 4)) (from versions: 3.4.10.37, 3.4.11.39, 3.4.11.41, 3.4.11.43, 3.4.11.45, 4.3.0.38, 4.4.0.40, 4.4.0.42, 4.4.0.44, 4.4.0.46)
ERROR: No matching distribution found for opencv-python==4.2.0.34 (from -r requirements.txt (line 4))

My setup:

$% sw_vers
ProductName:    macOS
ProductVersion: 11.1
BuildVersion:   20C5048k
$% python3 -V
Python 3.9.0
$% pip3 -V
pip 20.3.1 from /usr/local/lib/python3.9/site-packages/pip (python 3.9)

How can I fix this issue, so the installation is successful?

pip install --upgrade pip

didn't help.

soulisalmed / biff

Could not find a version that satisfies the requirement opencv-python #6