Use GH action version when version argument not specified

.git_archival.txt

@@ -0,0 +1,4 @@
+node: $Format:%H$
+node-date: $Format:%cI$
+describe-name: $Format:%(describe:tags=true,match=*[0-9]*)$
+ref-names: $Format:%D$

.gitattributes

@@ -0,0 +1 @@
+.git_archival.txt  export-subst

CHANGES.md

@@ -42,6 +42,9 @@
 
 <!-- For example, Docker, GitHub Actions, pre-commit, editors -->
 
+- Update GitHub Action to use the version of Black equivalent to action's version if
+  version input is not specified (#3543)
+
 ### Documentation
 
 <!-- Major changes to documentation and policies. Small docs changes

action/main.py

@@ -22,12 +22,34 @@
     extra_deps = "[colorama,jupyter]"
 else:
     extra_deps = "[colorama]"
-req = f"black{extra_deps}{version_specifier}"
+if version_specifier:
+    req = f"black{extra_deps}{version_specifier}"
+else:
+    describe_name = ""
+    with open(ACTION_PATH / ".git_archival.txt", encoding="utf-8") as fp:
+        for line in fp:
+            if line.startswith("describe-name: "):
+                describe_name = line[len("describe-name: ") :].rstrip()
+                break
+    if not describe_name:
+        print("::error::Failed to detect action version.", flush=True)
+        sys.exit(1)
+    # expected format is one of:
+    # - 23.1.0
+    # - 23.1.0-51-g448bba7
+    if describe_name.count("-") < 2:
+        # the action's commit matches a tag exactly, install exact version from PyPI
+        req = f"black{extra_deps}=={describe_name}"
+    else:
+        # the action's commit does not match any tag, install from the local git repo
+        req = f".{extra_deps}"
+print(f"Installing {req}...", flush=True)
 pip_proc = run(
     [str(ENV_BIN / "python"), "-m", "pip", "install", req],
     stdout=PIPE,
     stderr=STDOUT,
     encoding="utf-8",
+    cwd=ACTION_PATH,
 )
 if pip_proc.returncode:
     print(pip_proc.stdout)