Why do the installation of Sagemath fails on Ubuntu? I tried

jaakko@jaakko-Aspire-E1-572:~$ sudo apt install sagemath-common

[sudo] password for jaakko: 

Reading package lists... Done

Building dependency tree       

Reading state information... Done

The following NEW packages will be installed:

  sagemath-common

0 upgraded, 1 newly installed, 0 to remove and 8 not upgraded.

Need to get 16,2 MB of archives.

After this operation, 205 MB of additional disk space will be used.

Get:1 http://fi.archive.ubuntu.com/ubuntu focal/universe amd64 sagemath-common all 9.0-1ubuntu4 [16,2 MB]

Fetched 16,2 MB in 4s (3 888 kB/s)            

Selecting previously unselected package sagemath-common.

(Reading database ... 216481 files and directories currently installed.)

Preparing to unpack .../sagemath-common_9.0-1ubuntu4_all.deb ...

Unpacking sagemath-common (9.0-1ubuntu4) ...

Setting up sagemath-common (9.0-1ubuntu4) ...

/usr/lib/python3/dist-packages/sage/combinat/root_system/branching_rules.py:1753

: SyntaxWarning: "is not" with a literal. Did you mean "!="?

  if len(stypes) is not 2:

/usr/lib/python3/dist-packages/sage/graphs/graph_latex.py:1159: SyntaxWarning: '

str' object is not callable; perhaps you missed a comma?

  raise TypeError('%s option must be a dictionary, not %s' (name, value))

/usr/lib/python3/dist-packages/sage/graphs/graph_latex.py:1168: SyntaxWarning: '

str' object is not callable; perhaps you missed a comma?

  raise TypeError('%s option must be a dictionary, not %s' (name, value))

/usr/lib/python3/dist-packages/sage/graphs/graph_latex.py:1175: SyntaxWarning: '

str' object is not callable; perhaps you missed a comma?

  raise TypeError('%s option must be a dictionary, not %s' (name, value))

/usr/lib/python3/dist-packages/sage/graphs/graph_latex.py:1182: SyntaxWarning: '

str' object is not callable; perhaps you missed a comma?

  raise TypeError('%s option must be a dictionary, not %s' (name, value))

/usr/lib/python3/dist-packages/sage/graphs/graph_latex.py:1189: SyntaxWarning: '

str' object is not callable; perhaps you missed a comma?

  raise TypeError('%s option must be a dictionary, not %s' (name, value))

/usr/lib/python3/dist-packages/sage/graphs/graph_latex.py:1196: SyntaxWarning: '

str' object is not callable; perhaps you missed a comma?

  raise TypeError('%s option must be a dictionary, not %s' (name, value))

/usr/lib/python3/dist-packages/sage/graphs/graph_latex.py:1203: SyntaxWarning: '

str' object is not callable; perhaps you missed a comma?

  raise TypeError('%s option must be a dictionary, not %s' (name, value))

Processing triggers for man-db (2.9.1-1) ...

jaakko@jaakko-Aspire-E1-572:~$ sage

┌────────────────────────────────────────────────────────────────────┐

│ SageMath version 9.0, Release Date: 2020-01-01                     │

│ Using Python 3.8.5. Type "help()" for help.                        │

└────────────────────────────────────────────────────────────────────┘

Traceback (most recent call last):

  File "/usr/share/sagemath/bin/sage-ipython", line 12, in <module>

    from sage.repl.interpreter import SageTerminalApp

  File "/usr/lib/python3/dist-packages/sage/repl/interpreter.py", line 105, in <module>

    from sage.repl.preparse import preparse, containing_block

  File "/usr/lib/python3/dist-packages/sage/repl/preparse.py", line 227, in <module>

    from sage.repl.load import load_wrap

  File "/usr/lib/python3/dist-packages/sage/repl/load.py", line 19, in <module>

    from sage.cpython.string import str_to_bytes, bytes_to_str, FS_ENCODING

ModuleNotFoundError: No module named 'sage.cpython.string'

jaakko@jaakko-Aspire-E1-572:~$