Intermittant TabError: inconsistent use of tabs and spaces in indentation (only on CircleCI)

Question

I have just transitioned my program from python2 to python3. Everything checks out on my computer. When I run pytest, it works fine.

When I commit to github, and circleci test it, it get:

ImportError while loading conftest '/home/circleci/project/tests/conftest.py'.
tests/conftest.py:11: in <module>
    from auto_profiler import *
auto_profiler.py:4: in <module>
    from common import dbsettings
E     File "/home/circleci/project/common/dbsettings.py", line 23
E       params[k.lower()]=cgi.escape(v)
E                                     ^
E   TabError: inconsistent use of tabs and spaces in indentation

Again, it works fine when I run pytest on my home machine.

When I look through the file, I find no tabs.

Why is only circleci complaining? and where is the blasted tab?

ETA: This is a test which uses a sub-repo. My current theory is that circle is using the master branch of the sub-repo rather than the branch of the subRepo that it should be, (which SHOULD be committed to the master Repo branch, but maybe not?)

ETA2: My circleci has:

version: 2
jobs:
  build:
    docker:
      - image: circleci/python:3.7
    steps:
      - checkout
      - run:
          name: "Pull Submodules"
          command: |
            git submodule init
            git submodule update --remote

that means it's updating to master branch, right?