Fix mypy errors in lab.py

[tianyizheng02]

Run mypy with --strict flag and fix errors in lab.py due to a missing type hint

Contributes to #45 by fixing existing type hint errors prior to introducing mypy to the repo.

Before:

> mypy --strict pittapi/lab.py 
pittapi/lab.py:55: error: Function is missing a type annotation  [no-untyped-def]
pittapi/lab.py:84: error: Call to untyped function "LabAPIError" in typed context  [no-untyped-call]
pittapi/lab.py:86: error: Call to untyped function "LabAPIError" in typed context  [no-untyped-call]
Found 3 errors in 1 file (checked 1 source file)

After:

> mypy --strict pittapi/lab.py
Success: no issues found in 1 source file