build: Update lower bound of typing-extensions to v3.7.4.3 (#1961)

* Update lower bound of the supported typing-extensions versions to v3.7.4.3
  to ensure SupportsIndex is available, which is used as of PR #1940.
   - Amends PR #1940.
* Update tests/constraints.txt to use typing-extensions==3.7.4.3.

Author: matthewfeickert

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>=3.7.4; python_version == "3.7"  # for TypedDict, Literal
+    typing_extensions>=3.7.4.3; python_version == "3.7"  # for SupportsIndex
 
 [options.packages.find]
 where = src

tests/constraints.txt

@@ -6,7 +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
+typing-extensions==3.7.4.3  # c.f. PR #1961, #1940
 # xmlio
 uproot==4.1.1
 # minuit