xlrd.biffh.xlrderror: excel xlsx file; not supported

                                    As noted in the release email, linked to from the release tweet and noted in large orange warning that appears on the front page of the documentation, and less orange, but still present, in the readme on the repository and the release on pypi:

xlrd has explicitly removed support for anything other than xls files.

In your case, the solution is to:

make sure you are on a recent version of Pandas, at least 1.0.1, and preferably the latest release. 1.2 will make his even clearer.
install openpyxl: https://openpyxl.readthedocs.io/en/stable/
change your Pandas code to be:
df1 = pd.read_excel(
     os.path.join(APP_PATH, "Data", "aug_latest.xlsm"),
     engine='openpyxl',
)