fix(pacmak): python pack fails when installing 'black' via pip

The failure is because of intermittent unavailability of black or one of
its packages from the PyPI index.

Instead, install back into the environment, i.e., superchain, and use
that.

packages/jsii-pacmak/lib/targets/python.ts

@@ -45,29 +45,9 @@ export default class Python extends Target {
   public async generateCode(outDir: string, tarball: string): Promise<void> {
     await super.generateCode(outDir, tarball);
 
-    // We'll just run "black" on that now, to make the generated code a little more readable.
-    const blackRoot = await fs.mkdtemp(
-      path.join(os.tmpdir(), 'jsii-pacmak-black-'),
-    );
-    try {
-      await shell('python3', ['-m', 'venv', path.join(blackRoot, '.env')], {
-        cwd: blackRoot,
-      });
-      await shell(
-        path.join(blackRoot, '.env', 'bin', 'pip'),
-        ['install', 'black'],
-        { cwd: blackRoot },
-      );
-      await shell(
-        path.join(blackRoot, '.env', 'bin', 'black'),
-        ['--py36', outDir],
-        {
-          cwd: outDir,
-        },
-      );
-    } finally {
-      await fs.remove(blackRoot);
-    }
+    await shell('black', ['--py36', outDir], {
+      cwd: outDir,
+    });
   }
 
   public async build(sourceDir: string, outDir: string): Promise<void> {

superchain/Dockerfile

@@ -20,7 +20,7 @@ RUN rpm --import "https://packages.microsoft.com/keys/microsoft.asc"
 
 # Install Python 3
 RUN yum -y install python3 python3-pip                                                                                  \
-  && python3 -m pip install --upgrade pip setuptools wheel twine                                                        \
+  && python3 -m pip install --upgrade pip setuptools wheel twine black                                                  \
   && yum clean all && rm -rf /var/cache/yum
 
 # Install Ruby 2.6+