migrate the core of the code to 3.9 minimum and not 3.8

Author: kratsg

pyproject.toml

@@ -222,6 +222,7 @@ warn_unused_configs = true
 strict = true
 enable_error_code = ["ignore-without-code", "redundant-expr", "truthy-bool"]
 warn_unreachable = true
+plugins = "numpy.typing.mypy_plugin"
 
 [[tool.mypy.overrides]]
 module = [

src/pyhf/tensor/manager.py

@@ -65,6 +65,11 @@ def set_backend(
 
     Example:
         >>> import pyhf
+        >>> pyhf.set_backend(b"jax", precision="32b")
+        >>> pyhf.tensorlib.name
+        'jax'
+        >>> pyhf.tensorlib.precision
+        '32b'
         >>> pyhf.set_backend(pyhf.tensor.numpy_backend())
         >>> pyhf.tensorlib.name
         'numpy'

tests/conftest.py

@@ -89,7 +89,7 @@ def reset_backend():
 def backend(request):
     # a better way to get the id? all the backends we have so far for testing
     param_ids = request._fixturedef.ids
-    # the backend we're using: numpy, etc...
+    # the backend we're using: numpy, jax, etc...
     param_id = param_ids[request.param_index]
     # name of function being called (with params), the original name is .originalname
     func_name = request._pyfuncitem.name

tests/test_public_api.py

@@ -99,6 +99,7 @@ def test_custom_backend_name_supported():
     class custom_backend:
         def __init__(self, **kwargs):
             self.name = "jax"
+            self.name = "numpy"
             self.precision = '64b'
 
         def _setup(self):