flit_core >=3.9,<4 not support

[Sue1990]

When I was installing MHN, an error message appeared stating that the flit_core >=3.9,<4 package is not compatible. How can I resolve this issue?

Collecting git+https://github.com/defnull/bottle/ (from -r requirements.txt (line 5))
Cloning https://github.com/defnull/bottle/ to /tmp/pip-req-build-Prmiba
Running command git clone -q https://github.com/defnull/bottle/ /tmp/pip-req-build-Prmiba
Installing build dependencies ... error
ERROR: Command errored out with exit status 1:
command: /opt/mnemosyne/env/bin/python /opt/mnemosyne/env/lib/python2.7/site-packages/pip install --ignore-installed --no-user --prefix /tmp/pip-build-env-YsGsTB/overlay --no-warn-script-location --no-binary :none: --only-binary :none: -i https://pypi.org/simple -- 'flit_core >=3.9,<4'
cwd: None
Complete output (4 lines):
DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support pip 21.0 will remove support for this functionality.
WARNING: The directory '/home/malware/.cache/pip' or its parent directory is not owned or is not writable by the current user. The cache has been disabled. Check the permissions and owner of that directory. If executing pip with sudo, you may want sudo's -H flag.
ERROR: Could not find a version that satisfies the requirement flit_core<4,>=3.9 (from versions: 2.0rc1, 2.0rc2, 2.0rc3, 2.0, 2.0.1, 2.0.2, 2.1.0, 2.2.0, 2.3.0)
ERROR: No matching distribution found for flit_core<4,>=3.9

ERROR: Command errored out with exit status 1: /opt/mnemosyne/env/bin/python /opt/mnemosyne/env/lib/python2.7/site-packages/pip install --ignore-installed --no-user --prefix /tmp/pip-build-env-YsGsTB/overlay --no-warn-script-location --no-binary :none: --only-binary :none: -i https://pypi.org/simple -- 'flit_core >=3.9,<4' Check the logs for full command output.