build: Set lower bound of typing-extensions to v3.7.4 (#1938)

* Set the lower bound of the supported typing_extensions versions to v3.7.4
  as TypedDict was added in v3.7.4.

  > The typing_extensions module contains both backports of these changes as
  > well as experimental types that will eventually be added to the typing module,
  > such as Protocol or TypedDict.
   - c.f. https://pypi.org/project/typing-extensions/3.7.4/

   - Amends PR #1934
* Add typing-extensions==3.7.4 to tests/constraints.txt.

setup.cfg

@@ -42,7 +42,7 @@ install_requires =
     jsonpatch>=1.15
     pyyaml>=5.1  # for parsing CLI equal-delimited options
     importlib_resources>=1.3.0; python_version < "3.9"  # for resources in schema
-    typing_extensions; python_version == "3.7"  # for TypedDict, Literal
+    typing_extensions>=3.7.4; python_version == "3.7"  # for TypedDict, Literal
 
 [options.packages.find]
 where = src

tests/constraints.txt

@@ -6,6 +6,7 @@ jsonschema==3.0.0
 jsonpatch==1.15
 pyyaml==5.1
 importlib_resources==1.3.0
+typing-extensions==3.7.4  # c.f. PR #1938
 # xmlio
 uproot==4.1.1
 # minuit