Deploy to pypi fails for python 3.5

I am using travis for automatic package deployment to pypi upon new version release. In my latest release, deployment failed for python 3.5 under OSX and Linux. It appears to be a pip related issue.

Here is the build.

And here is my travis config file.

The error message is

Fetching: dpl-pypi-1.10.14.gem (100%)
Successfully installed dpl-pypi-1.10.14
1 gem installed
2019-12-11 00:24:40 URL: [1775835/1775835] -> "-" [1]
Collecting pip
  Downloading (1.4MB)
     |████████████████████████████████| 1.4MB 15.3MB/s 
Installing collected packages: pip
Successfully installed pip-19.3.1
Traceback (most recent call last):
  File "/Users/travis/miniconda/envs/test_env/bin/pip", line 11, in <module>
TypeError: 'module' object is not callable
Couldn't install pip, setuptools, twine or wheel.
failed to deploy

Looking up the error, there seems to be some work around, but I’m not sure how that would work on Travis.

Thank you very much for the pointer!