Use `unittest.mock` if it is available

[major]

Environment details

• OS type and version: Fedora 34
• Python version: Python 3.9.5
• pip version: pip 21.1.2
• google-api-core version: 1.3.0

Using the mock module for recent versions of Python is deprecated. If the built-in unittest.mock is available for testing, that should be used instead.

[tseaver]

Superseded by #210.