bpo-37885: venv: Don't produce unbound variable warning on deactivate (GH-15973)

Before, running deactivate from a bash shell configured to treat undefined variables as errors (`set -u`) would produce a warning:

```
$ python3 -m venv test
$ source test/bin/activate
(test) $ deactivate
-bash: $1: unbound variable
```
(cherry picked from commit 5209e586b7)

Co-authored-by: Daniel Abrahamsson <hamsson@gmail.com>

Lib/test/test_venv.py

@@ -9,6 +9,7 @@ import ensurepip
 import os
 import os.path
 import re
+import shutil
 import struct
 import subprocess
 import sys
@@ -338,6 +339,25 @@ class BasicTest(BaseTest):
             'pool.terminate()'])
         self.assertEqual(out.strip(), "python".encode())
 
+    @unittest.skipIf(os.name == 'nt', 'not relevant on Windows')
+    def test_deactivate_with_strict_bash_opts(self):
+        bash = shutil.which("bash")
+        if bash is None:
+            self.skipTest("bash required for this test")
+        rmtree(self.env_dir)
+        builder = venv.EnvBuilder(clear=True)
+        builder.create(self.env_dir)
+        activate = os.path.join(self.env_dir, self.bindir, "activate")
+        test_script = os.path.join(self.env_dir, "test_strict.sh")
+        with open(test_script, "w") as f:
+            f.write("set -euo pipefail\n"
+                    f"source {activate}\n"
+                    "deactivate\n")
+        out, err = check_output([bash, test_script])
+        self.assertEqual(out, "".encode())
+        self.assertEqual(err, "".encode())
+
+
 @requireVenvCreate
 class EnsurePipTest(BaseTest):
     """Test venv module installation of pip."""