Fix papermill

[Philmod]

The default python version was not the one on which everything was installed.

Added a unit test that fails with gcr.io/kaggle-images/rstats:v37:

── Warning (test_papermill.R:15:2): python papermill exists ─────────────────────
running command 'python -c 'import sys;import papermill as pm; print(pm.__version__)'' had status 1
Backtrace:
 1. testthat::expect_error(...) test_papermill.R:15:8
 6. base::system(...) test_papermill.R:16:16

── Failure (test_papermill.R:15:2): python papermill exists ───────────────────────────
`{ ... }` threw an error.
Message: `res` is empty.
Class:   expectation_failure/expectation/error/condition
Backtrace:
 1. testthat::expect_error(...) test_papermill.R:15:8
 6. testthat::expect_match(res, "\\d\\.\\d\\.\\d") test_papermill.R:18:16

[ FAIL 1 | WARN 1 | SKIP 0 | PASS 2 ]

but succeeds with this change:

[ FAIL 0 | WARN 0 | SKIP 0 | PASS 4 ] Done!

http://b/191304257