Close key file after reading in Python example

[bejota]

Don't open a file in Python without closing it. This code is bad mojo:

app = msal.ConfidentialClientApplication(
    config["client_id"], authority=config["authority"],
    client_credential={"thumbprint": config["thumbprint"], "private_key": open(config['private_key_file']).read()},
    )

Prefer:

with open(config['private_key_file']) as f:
        key = f.read()
app = msal.ConfidentialClientApplication(
    config["client_id"], authority=config["authority"],
    client_credential={"thumbprint": config["thumbprint"], "private_key": key },
    )