Unit tests check fails on Python 2.7 and Python 3.5, because not all dependencies can be installed.

Environment details

• OS type and version: Linux (and possibly others?)
• Python version: 2.7, 3.5
• pip version: pip --version: 20.0.2
• google-cloud-bigquery version: 1.24.0

Steps to reproduce

1. Run uni tests session for Python 2.7 or 3.5, e.g.:
   nox -f noxfile.py -s unit-2.7
2. Test do not run, an error occurs when installing dependencies.

Code example

# example

Stack trace

Building wheels for collected packages: llvmlite
...
RuntimeError: Building llvmlite requires LLVM 7.0.x, 7.1.x or 8.0.x, got '11.0.0'. Be sure to set LLVM_CONFIG to the right executable path.