Disable dev_appserver.py usage

[MontyCarter]

It seems like github actions switched to python3 because devappserver.py no longer runs, and complains with python2 not found.

https://github.blog/changelog/2020-02-27-github-actions-breaking-change-python-2-being-removed-from-all-virtual-environments/

actions/setup-python#543 (comment)

python2 support will not be added to ubuntu 22.04, it seems.

[jinglundong]

I couldn't tell for sure if we use dev_appserver.py in other places in this repo. From my scan of https://github.com/golang/appengine/search?p=1&q=dev_appserver, this PR is okay.

I approved the workflow run (because this is your first PR I assume). Let's see if workflows pass this time.

[jinglundong]

Tests failed. @MontyCarter

[jinglundong]

It's still dev_appserver related.

=== RUN   TestBasicAPICalls
ERROR: (dev_appserver) python2: command not found
    instance_test.go:24: NewInstance: unable to find admin server URL
--- FAIL: TestBasicAPICalls (1.63s)
=== RUN   TestContext
ERROR: (dev_appserver) python2: command not found
    instance_test.go:75: NewContext: unable to find admin server URL
--- FAIL: TestContext (0.46s)
=== RUN   TestUsers
ERROR: (dev_appserver) python2: command not found
    instance_test.go:88: NewInstance: unable to find admin server URL
--- FAIL: TestUsers (0.47s)
FAIL
coverage: 41.6% of statements
FAIL	google.golang.org/appengine/aetest	2.595s

[ludoch]

I would rather try to make devappserver work again.

IIUC, the new base image used for tests does not include python2, so we need to install it so it can be used by the devappserver.

[ludoch]

Not needed anymore if we merge #304