language/python: support python3 from PATH in #detected_python_shebang

Some formulae are flexible about the version of Python3 that they use.
However, when we use `#detected_python_shebang` on these formulae, they
become coupled to the specific version of Python3 declared in the
formula.

This is harmful because
1. it prevents us from using `uses_from_macos "python"` even in formulae
   where we should be able to
2. it forces us to rebuild the formula whenever we make changes to the
   Python dependency when nothing but the shebang would have changed as
   a consequence of the rebuild

For an example of this, see Homebrew/homebrew-core#107905.

I'd also like to do this to get rid of some really terrible hacks we
have in `glib-utils` as a means of decoupling `glib` from the specific
versioned Python dependency it used to have.

See Homebrew/homebrew-core#103916, or Homebrew/homebrew-core#106045 for
a proposal to give the same treatment to `gobject-introspection`.

Library/Homebrew/language/python.rb

@@ -103,16 +103,22 @@ module Language
         )
       end
 
-      def detected_python_shebang(formula = self)
-        python_deps = formula.deps.map(&:name).grep(/^python(@.*)?$/)
+      def detected_python_shebang(formula = self, use_python_from_path: false)
+        python_path = if use_python_from_path
+          "/usr/bin/env python3"
+        else
+          python_deps = formula.deps.map(&:name).grep(/^python(@.*)?$/)
 
-        raise ShebangDetectionError.new("Python", "formula does not depend on Python") if python_deps.empty?
-        if python_deps.length > 1
-          raise ShebangDetectionError.new("Python", "formula has multiple Python dependencies")
+          raise ShebangDetectionError.new("Python", "formula does not depend on Python") if python_deps.empty?
+          if python_deps.length > 1
+            raise ShebangDetectionError.new("Python", "formula has multiple Python dependencies")
+          end
+
+          python_dep = python_deps.first
+          Formula[python_dep].opt_bin/python_dep.sub("@", "")
         end
 
-        python_dep = python_deps.first
-        python_shebang_rewrite_info(Formula[python_dep].opt_bin/python_dep.sub("@", ""))
+        python_shebang_rewrite_info(python_path)
       end
     end
 

Library/Homebrew/test/language/python/shebang_spec.rb

@@ -21,7 +21,7 @@ describe Language::Python::Shebang do
 
   before do
     file.write <<~EOS
-      #!/usr/bin/env python3
+      #!/usr/bin/python2
       a
       b
       c
@@ -34,7 +34,7 @@ describe Language::Python::Shebang do
   describe "#detected_python_shebang" do
     it "can be used to replace Python shebangs" do
       expect(Formulary).to receive(:factory).with(python_f.name).and_return(python_f)
-      Utils::Shebang.rewrite_shebang described_class.detected_python_shebang(f), file
+      Utils::Shebang.rewrite_shebang described_class.detected_python_shebang(f, use_python_from_path: false), file
 
       expect(File.read(file)).to eq <<~EOS
         #!#{HOMEBREW_PREFIX}/opt/python@3.11/bin/python3.11
@@ -43,5 +43,16 @@ describe Language::Python::Shebang do
         c
       EOS
     end
+
+    it "can be pointed to a `python3` in PATH" do
+      Utils::Shebang.rewrite_shebang described_class.detected_python_shebang(f, use_python_from_path: true), file
+
+      expect(File.read(file)).to eq <<~EOS
+        #!/usr/bin/env python3
+        a
+        b
+        c
+      EOS
+    end
   end
 end