[python] Use pip --upgrade when installing function dependencies (#6683)

Co-authored-by: Steven <steven@ceriously.com>

packages/python/src/install.ts

@@ -86,6 +86,10 @@ interface InstallRequirementArg {
   args?: string[];
 }
 
+// note that any internal dependency that vc_init.py requires that's installed
+// with this function can get overriden by a newer version from requirements.txt,
+// so vc_init should do runtime version checks to be compatible with any recent
+// version of its dependencies
 export async function installRequirement({
   dependency,
   version,
@@ -120,5 +124,5 @@ export async function installRequirementsFile({
     debug(`Skipping requirements file installation, already installed`);
     return;
   }
-  await pipInstall(workPath, ['-r', filePath, ...args]);
+  await pipInstall(workPath, ['--upgrade', '-r', filePath, ...args]);
 }

packages/python/test/fixtures/29-flask2/api/werkzeug_version.py

@@ -0,0 +1,11 @@
+import werkzeug
+from flask import Flask
+app = Flask(__name__)
+
+@app.route('/', defaults={'path': ''})
+@app.route('/<path:path>')
+def main(path):
+    return werkzeug.__version__
+
+if __name__ == '__main__':
+    app.run(debug=True, port=8002)

packages/python/test/fixtures/29-flask2/requirements.txt

@@ -0,0 +1,2 @@
+flask==2.0.1
+werkzeug==2.0.1

packages/python/test/fixtures/29-flask2/vercel.json

@@ -0,0 +1,10 @@
+{
+  "version": 2,
+  "builds": [{ "src": "api/*.py", "use": "@vercel/python" }],
+  "probes": [
+    {
+      "path": "/api/werkzeug_version.py",
+      "mustContain": "2.0.1"
+    }
+  ]
+}