This is an adapted patch from the games/anki FreeBSD port.
https://github.com/freebsd/freebsd-ports/blob/109c3d4629b84972e660b689d169ac0761c1a519/games/anki/files/patch-build_configure_src_python.rs
https://github.com/freebsd/freebsd-ports/blob/109c3d4629b84972e660b689d169ac0761c1a519/games/anki/files/patch-build_ninja__gen_src_python.rs

Use the "PythonEnvironmentStub" function to use a native Python environment
instead of a Python venv.
* Avoid the use of Python's pip (network access).
* Remove the python_binary input for PythonEnvironmentStub since we create the
  pseudo venv manually in the ebuild. This saves us from setting an extra
  environment variable to prevent network access.
  
  The python_binary input should really only be added to PythonEnvironment, if
  the venv is not set up.
* TODO: add dev-python/mypy-protobuf to tree for fully typed Anki.

From: Lucio Sauer <watermanpaint@posteo.net>
--- a/build/configure/src/python.rs
+++ b/build/configure/src/python.rs
@@ -13,6 +13,7 @@ use ninja_gen::input::BuildInput;
 use ninja_gen::inputs;
 use ninja_gen::python::python_format;
 use ninja_gen::python::PythonEnvironment;
+use ninja_gen::python::PythonEnvironmentStub;
 use ninja_gen::python::PythonLint;
 use ninja_gen::python::PythonTypecheck;
 use ninja_gen::rsync::RsyncFiles;
@@ -81,6 +82,25 @@ pub fn setup_venv(build: &mut Build) -> Result<()> {
     Ok(())
 }
 
+pub fn setup_venv_stub(build: &mut Build) -> Result<()> {
+    build.add_action(
+        "pyenv",
+        PythonEnvironmentStub {
+            folder: "pyenv",
+            extra_binary_exports: &[
+                "mypy",     // Required in some parts of the code, but not for build
+                "black",    // dito
+                "isort",    // dito
+                "pylint",   // dito
+                "pytest",   // dito
+                "protoc-gen-mypy",
+            ],
+        },
+    )?;
+
+    Ok(())
+}
+
 pub struct GenPythonProto {
     pub proto_files: BuildInput,
 }
@@ -88,9 +108,7 @@ pub struct GenPythonProto {
 impl BuildAction for GenPythonProto {
     fn command(&self) -> &str {
         "$protoc $
-        --plugin=protoc-gen-mypy=$protoc-gen-mypy $
         --python_out=$builddir/pylib $
-        --mypy_out=$builddir/pylib $
         -Iproto $in"
     }
 
@@ -108,7 +126,6 @@ impl BuildAction for GenPythonProto {
             .collect();
         build.add_inputs("in", &self.proto_files);
         build.add_inputs("protoc", inputs![":protoc_binary"]);
-        build.add_inputs("protoc-gen-mypy", inputs![":pyenv:protoc-gen-mypy"]);
         build.add_outputs("", python_outputs);
     }
 
@@ -254,7 +271,6 @@ impl BuildAction for Sphinx {
 
     fn files(&mut self, build: &mut impl FilesHandle) {
         build.add_inputs("python", inputs![":pyenv:bin"]);
-        build.add_inputs("pip", inputs![":pyenv:pip"]);
         build.add_inputs("", &self.deps);
         build.add_output_stamp("python/sphinx/stamp");
     }
--- a/build/ninja_gen/src/python.rs
+++ b/build/ninja_gen/src/python.rs
@@ -86,6 +86,11 @@ pub struct PythonEnvironment {
     pub extra_binary_exports: &'static [&'static str],
 }
 
+pub struct PythonEnvironmentStub {
+    pub folder: &'static str,
+    pub extra_binary_exports: &'static [&'static str],
+}
+
 impl BuildAction for PythonEnvironment {
     fn command(&self) -> &str {
         "$runner pyenv $python_binary $builddir/$pyenv_folder $system_pkgs $base_requirements $requirements"
@@ -118,6 +123,34 @@ impl BuildAction for PythonEnvironment {
     }
 }
 
+impl BuildAction for PythonEnvironmentStub {
+    fn command(&self) -> &str {
+        "echo Running PythonEnvironmentStub..."
+    }
+
+    fn files(&mut self, build: &mut impl crate::build::FilesHandle) {
+        let bin_path = |binary: &str| -> Vec<String> {
+            let folder = self.folder;
+            let path = if cfg!(windows) {
+                format!("{folder}/scripts/{binary}.exe")
+            } else {
+                format!("{folder}/bin/{binary}")
+            };
+            vec![path]
+        };
+
+        build.add_variable("pyenv_folder", self.folder);
+        build.add_outputs_ext("bin", bin_path("python"), true);
+        for binary in self.extra_binary_exports {
+            build.add_outputs_ext(*binary, bin_path(binary), true);
+        }
+    }
+
+    fn check_output_timestamps(&self) -> bool {
+        true
+    }
+}
+
 pub struct PythonTypecheck {
     pub folders: &'static [&'static str],
     pub deps: BuildInput,