[python] Fix error message for discontinued Python 3.6 (#8300)

This fixes the error message when a discontinued version of python (for example, Python 3.6) is detected.

https://vercel.com/changelog/python-3-6-is-being-deprecated

packages/python/src/index.ts

@@ -109,20 +109,28 @@ export const build = async ({
   if (pipfileLockDir) {
     debug('Found "Pipfile.lock"');
 
+    let lock: {
+      _meta?: {
+        requires?: {
+          python_version?: string;
+        };
+      };
+    } = {};
     try {
       const json = await readFile(join(pipfileLockDir, 'Pipfile.lock'), 'utf8');
-      const obj = JSON.parse(json);
-      pythonVersion = getSupportedPythonVersion({
-        isDev: meta.isDev,
-        pipLockPythonVersion: obj?._meta?.requires?.python_version,
-      });
+      lock = JSON.parse(json);
     } catch (err) {
       throw new NowBuildError({
         code: 'INVALID_PIPFILE_LOCK',
         message: 'Unable to parse Pipfile.lock',
       });
     }
 
+    pythonVersion = getSupportedPythonVersion({
+      isDev: meta.isDev,
+      pipLockPythonVersion: lock?._meta?.requires?.python_version,
+    });
+
     // Convert Pipenv.Lock to requirements.txt.
     // We use a different`workPath` here because we want `pipfile-requirements` and it's dependencies
     // to not be part of the lambda environment. By using pip's `--target` directive we can isolate

packages/python/test/fixtures/30-fail-build-invalid-pipfile/Pipfile

@@ -0,0 +1 @@
+blarg

packages/python/test/fixtures/30-fail-build-invalid-pipfile/api/index.py

@@ -0,0 +1,10 @@
+from http.server import BaseHTTPRequestHandler, HTTPServer
+
+class handler(BaseHTTPRequestHandler):
+
+    def do_GET(self):
+        self.send_response(200)
+        self.send_header('Content-type','text/plain')
+        self.end_headers()
+        self.wfile.write(self.path.encode())
+        return

packages/python/test/fixtures/30-fail-build-invalid-pipfile/vercel.json

@@ -0,0 +1 @@
+{}

packages/python/test/fixtures/31-fail-build-invalid-python36/Pipfile

@@ -0,0 +1,12 @@
+[[source]]
+name = "pypi"
+url = "https://pypi.org/simple"
+verify_ssl = true
+
+[dev-packages]
+
+[packages]
+cowpy = "*"
+
+[requires]
+python_version = "3.6"

packages/python/test/fixtures/31-fail-build-invalid-python36/api/index.py

@@ -0,0 +1,13 @@
+from http.server import BaseHTTPRequestHandler
+from cowpy import cow
+
+
+class handler(BaseHTTPRequestHandler):
+
+    def do_GET(self):
+        self.send_response(200)
+        self.send_header('Content-type', 'text/plain')
+        self.end_headers()
+        message = cow.Cowacter().milk('pip:RANDOMNESS_PLACEHOLDER')
+        self.wfile.write(message.encode())
+        return

packages/python/test/fixtures/31-fail-build-invalid-python36/vercel.json

@@ -0,0 +1 @@
+{}

packages/python/test/integration.test.ts

@@ -71,8 +71,30 @@ it('should match the probes against Python dev servers', async () => {
   }
 });
 
+const testsThatFailToBuild = new Map([
+  ['30-fail-build-invalid-pipfile', 'Unable to parse Pipfile.lock'],
+  [
+    '31-fail-build-invalid-python36',
+    'Python version "3.6" detected in Pipfile.lock is discontinued and must be upgraded.',
+  ],
+]);
+
 // eslint-disable-next-line no-restricted-syntax
 for (const fixture of fs.readdirSync(fixturesPath)) {
+  const errMsg = testsThatFailToBuild.get(fixture);
+  if (errMsg) {
+    // eslint-disable-next-line no-loop-func
+    it(`should fail to build ${fixture}`, async () => {
+      try {
+        await testDeployment(path.join(fixturesPath, fixture));
+      } catch (err) {
+        expect(err).toBeTruthy();
+        expect(err.deployment).toBeTruthy();
+        expect(err.deployment.errorMessage).toBe(errMsg);
+      }
+    });
+    continue; //eslint-disable-line
+  }
   // eslint-disable-next-line no-loop-func
   it(`should build ${fixture}`, async () => {
     await expect(