Environment details

• OS: Linux
• Python version: 3.8.2
• pip version: 20.0.2
• google-auth version: 1.12.0

Steps to reproduce

1. Create an ID token using the GCE metadata server:

   from google.auth import compute_engine
   import google.auth.transport.requests
   request = google.auth.transport.requests.Request()
   target_audience = 'https://example.com'
   creds = compute_engine.IDTokenCredentials(request, target_audience, use_metadata_identity_endpoint=True)

2. Try to use the credentials to authorize a request:

   from google.auth.transport.requests import AuthorizedSession
   session = AuthorizedSession(credentials)
   response = session.get('https://example.com')

This results in an error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.8/site-packages/google/auth/credentials.py", line 66, in expired
    skewed_expiry = self.expiry - _helpers.CLOCK_SKEW
TypeError: unsupported operand type(s) for -: 'int' and 'datetime.timedelta'

I think the issue is on this line. payload["exp"] should (apparently) be getting parsed into a datetime.timedelta