fix: Add TYPE_CHECKING guard for numpy.typing (#2208)

* The addition of typing.TYPE_CHECKING avoids situations in which
  the version of NumPy (whose lower bounds is enforced by SciPy)
  doesn't have numpy.typing, which was added in NumPy v1.21.0.
* Exclude lines that match 'if TYPE_CHECKING:' from the coverage
  report to avoid artificial drops in code coverage.
   - c.f. https://coverage.readthedocs.io/en/stable/excluding.html#advanced-exclusion

Co-authored-by: Giordon Stark <kratsg@gmail.com>

pyproject.toml

@@ -225,6 +225,9 @@ omit = ["*/pyhf/typing.py"]
 precision = 1
 sort = "cover"
 show_missing = true
+exclude_also = [
+    "if TYPE_CHECKING:"
+]
 
 [tool.mypy]
 files = "src"

src/pyhf/tensor/numpy_backend.py

@@ -2,10 +2,16 @@
 from __future__ import annotations
 
 import logging
-from typing import Callable, Generic, Mapping, Sequence, TypeVar, Union
+from typing import TYPE_CHECKING, Callable, Generic, Mapping, Sequence, TypeVar, Union
 
 import numpy as np
-from numpy.typing import ArrayLike, DTypeLike, NBitBase, NDArray
+
+# Needed while numpy lower bound is older than v1.21.0
+if TYPE_CHECKING:
+    from numpy.typing import ArrayLike, DTypeLike, NBitBase, NDArray
+else:
+    NBitBase = "NBitBase"
+
 from scipy import special
 from scipy.special import gammaln, xlogy
 from scipy.stats import norm, poisson