Code isn't python 3.9 compatible - add backwards compatibility or drop support?

There are maybe other reasons for the code to not be backwards compatible with python 3.9, but the first one that arises when trying to run python -m httomo --help is the import TypeAlias import that is in yaml_checker.py: https://github.com/DiamondLightSource/httomo/blob/4429711ab89ed1efe86d9c3be260df25fc7f1999/httomo/yaml_checker.py#L6

This is a module that was introduced in python 3.10, and so naturally the import fails in python 3.9:

[twi18192@pc0074 ~]$ module load python/3.9

    Loading 64-bit Anaconda Python3, version 3.9
    If there are any problems, please contact dataanalysis@diamond.ac.uk

    Need a 3rd party Python package installing? First, consider using the
    user-based install option of pip to test it. (I.e., use
    $ pip install --user ... For further details, see $ pip help install)

    This will install the package in ~/.local
    Please read https://confluence.diamond.ac.uk/x/rKDIAw for other options.
    If a site-wide installation is required, contact Data Analysis.
    For further details, see $ pip help install

Loading python/3.9
  Loading requirement: global/directories
(python3.9) [twi18192@pc0074 ~]$ python --version
Python 3.9.1
(python3.9) [twi18192@pc0074 ~]$ python
Python 3.9.1 | packaged by conda-forge | (default, Jan 26 2021, 01:34:10)
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import TypeAlias
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: cannot import name 'TypeAlias' from 'typing' (/dls_sw/apps/python/anaconda/4.6.14/64/envs/python3.9/lib/python3.9/typing.py)

but works in python 3.10:

[twi18192@pc0074 ~]$ module load python/3.10

    Loading a conda environment for Python 3.10
    ***********************************************************
    This is now the default Python environment
    If there are any problems or packages missing compared with
    Python 3.7, please contact dataanalysis@diamond.ac.uk
    ***********************************************************

    Need a 3rd party Python package installing? First, consider using the
    user-based install option of pip to test it. (I.e., use
    $ pip install --user ... For further details, see $ pip help install)

    This will install the package in ~/.local
    Please read https://confluence.diamond.ac.uk/x/rKDIAw for other options.
    If a site-wide installation is required, contact Data Analysis.

Loading python/3.10
  Loading requirement: global/directories
(python3.10) [twi18192@pc0074 ~]$ python --version
Python 3.10.12
(python3.10) [twi18192@pc0074 ~]$ python
Python 3.10.12 | packaged by conda-forge | (main, Jun 23 2023, 22:40:32) [GCC 12.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import TypeAlias
>>> TypeAlias
typing.TypeAlias

There are several places in the repo where we cite python 3.9 compatibility, such as in the conda_build_config.yaml (used when building the conda package): https://github.com/DiamondLightSource/httomo/blob/4429711ab89ed1efe86d9c3be260df25fc7f1999/conda/recipe/conda_build_config.yaml#L1-L3

and pyproject.toml (used in both pip install and conda install): https://github.com/DiamondLightSource/httomo/blob/4429711ab89ed1efe86d9c3be260df25fc7f1999/pyproject.toml#L45 https://github.com/DiamondLightSource/httomo/blob/4429711ab89ed1efe86d9c3be260df25fc7f1999/pyproject.toml#L48 https://github.com/DiamondLightSource/httomo/blob/4429711ab89ed1efe86d9c3be260df25fc7f1999/pyproject.toml#L102

DiamondLightSource / httomo

Code isn't python 3.9 compatible - add backwards compatibility or drop support? #345