DEV-2479: py38 upgrade

[jiakf]

This PR also includes running pyupgrade for py38 on the repository, so most of the file changes are from that. The main fixes in this PR:

• For macOS, py38 makes spawn the default method for multiprocessing as opposed to fork, so the code as written no longer worked for macOS. For now, we are switching DTT to use threads on macOS as opposed to processes, so the same as what is done on Windows. Some performance testing was done to compare the threads vs process versions of macOS DTT, and the performance was comparable.
• Dropped some ubuntu/macOS versions from github actions as they are no longer supported.
• macOS takes longer to start up a process now, and so a race condition was happening in the conftest file when running the Flask app in a separate process. Increasing the sleep time after starting the new process gave enough time for the app to come up before trying to make requests
• the target function for Process also needs to be in the top-level module scope, so had to create a helper module function in conftest to start the Flask app.
• Used the fixes in DEV 2469 fix github action #286 to fix the github actions workflows
• FIxed github actions checkout step where setuptools_scm doesn't have the full git history to figure out the version

[mpsolano]

Looks good!

A lot has changed

[wrkhenddher]

Is this right?
isn't importlib_metadata already in py3.8? (so this should not be needed?)

https://docs.python.org/3/library/importlib.metadata.html#module-importlib.metadata

[jiakf]

i see that now, let me try building without importlib_metadata included in setup.py

[jiakf]

So the importlib library was made part of python3.8, but the syntax will have to change where it is used in this repo. Additionally, the features of importlib_metadata that were included in python3.8 are equivalent to importlib_metadata version 1, whereas this was already tested with importlib_metadata version 7.0.1. I'd recommend keeping the backport library in here as a result.

[stilesj-uchicago]

You sure you want to support any greater than 8? Like version 9 when it comes out? Or do you want ~=8.0?

[jiakf]

i think in the future if a newer version breaks functionality we can pin it then.

[fgvieira]

Any idea when this PR will be merged? Current bioconda recipe is broken and this PR might fix it:

[...]
  File "lib/python3.7/site-packages/python_utils/converters.py", line 89
    if match := regexp.search(input_):
              ^
SyntaxError: invalid syntax