fix: use 3.5 version of get-pip.py

[icywang86rui]

Issue #, if available:
The multimodel tests are failing because they failed to build the image needed in the tests. The base image is ubuntu16 so the default python3 version installed is 3.5 and it doesn't support f string which is used in the new version of get-pip.py:

  File "get-pip.py", line 82, in bootstrap
    from pip._internal.cli.main import main as pip_entry_point
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 954, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 896, in _find_spec
  File "<frozen importlib._bootstrap_external>", line 1139, in find_spec
  File "<frozen importlib._bootstrap_external>", line 1115, in _get_spec
  File "<frozen importlib._bootstrap_external>", line 1096, in _legacy_get_spec
  File "<frozen importlib._bootstrap>", line 444, in spec_from_loader
  File "<frozen importlib._bootstrap_external>", line 533, in spec_from_file_location
  File "/tmp/tmpxa_sx1lv/pip.zip/pip/_internal/cli/main.py", line 60
    sys.stderr.write(f"ERROR: {exc}")
                                   ^
SyntaxError: invalid syntax

This change should fix the problem. I just tried building this dockerfile locally.

Description of changes:

Testing done:

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