The following code generates a path that works in Pythons 3.7.6 on macOS Big Sur but not in Catalina  with 3.8.2.. 
mySrcFldr="~/Library/Mobile Documents/com~apple~CloudDocs/Utilities/"
srcFldr=os.path.expanduser(mySrcFldr)
f=os.path.join(srcFldr,"mortgage.py")

When python tries to run the code file(montage.py) on 3.8.2, an open error results.
/Library/Developer/CommandLineTools/usr/bin/python3: can't open file '/Users/Doug/Library/Mobile': [Errno 2] No such file or directory
 I tried putting a backslash after 'Mobile'. Does not help.
The complete path is:
'/Users/Doug/Library/Mobile Documents/com~apple~CloudDocs/Utilities/mortgage.py'

While the doc changes show some changes in the os.path module, I see no mention of this issue.

To clarify: either python version generates the same path. On 3.8.2 though an open error results

What kind of error do you get?

Please provide complete script that reproduces the issue. The provided three lines do not open any file and cannot generate such error.

It could help to add the debug prints:

print(f'mySrcFldr = {mySrcFldr!a}')
print(f'srcFldr = {srcFldr!a}')
print(f'f = {f!a}')

This might be a security feature in Big Sur, access to some locations is restricted by default. I'm not sure if that includes ~/Library, it definitely affects access to ~/Documents. I'd expect to see a security pop-up from the system though.

I cannot reproduce this with python 3.9.1 on Big Sur on an M1 system.