Support copying files to a directory

[jerrykan]

This restores the ability for the copy() function to copy files to a directory. The changes are essentially the same as what is used in the Python standard library.

Pull Request Checklist

Resolves: #3442

• Ensured tests pass and (if applicable) updated functional test output
• Conformed to code style guidelines by running appropriate linting tools
• Added tests for changed code
• Updated documentation for changed code

[justinmayer]

Hi John. Thank you for the submission. In order to prevent future code modifications from causing regressions and breaking the behavior you are enabling here, could you add a test that ensures your target use case is covered?

[jerrykan]

Hi Justin I've added a test to cover copying a file to and existing directory.

While creating the test I noticed a possible edge-case where if copy() is used to try to copy a file to a directory that does not exist (ie. a/a.txt -> b0/b1/). If the b0/b1/ directory does not exist, then the file will be copied to b0/b1 (where b1 is a file).

There is no equivalent at the OS level (because copying a file wouldn't automatically create any directories) so the expected behavior is probably undefined. I'm not sure if this is an edge case that is worth worrying about.