Encountered an error on Python 3.9 - AttributeError: module 'base64' has no attribute 'decodestring'

Microsoft's app center has recently updated to Python 3.9, and I encountered the follow error when running gsutil on it:

```
Traceback (most recent call last):
File "/usr/local/bin/gsutil", line 5, in
from gslib.__main__ import main
File "/usr/local/lib/python3.9/site-packages/gslib/__main__.py", line 92, in
from gslib.command_runner import CommandRunner
File "/usr/local/lib/python3.9/site-packages/gslib/command_runner.py", line 64, in
from gslib.tests.util import HAS_NON_DEFAULT_GS_HOST
File "/usr/local/lib/python3.9/site-packages/gslib/tests/util.py", line 126, in
TEST_ENCRYPTION_KEY1_SHA256_B64 = Base64Sha256FromBase64EncryptionKey(
File "/usr/local/lib/python3.9/site-packages/gslib/utils/encryption_helper.py", line 152, in Base64Sha256FromBase64EncryptionKey
decoded_bytes = base64.decodestring(csek_encryption_key)
AttributeError: module 'base64' has no attribute 'decodestring'
```

This error is triggered with `gsutil -o Credentials:gs_service_key_file=./mykey.json cp src_url dist_url` command.

Any thoughts on how to get this command work on Python 3.9? Thank you.

Start in gslib/utils/encryption_helper.py at Base64Sha256FromBase64EncryptionKey, where the Python 3.9 traceback points to the failing call. Reproduce the issue with the reported gsutil command under Python 3.9 and verify that it completes without the AttributeError.

Written by the indexing model from the issue text.