When using tox and python 3.5.3 fail to build package

[kniklas]

To reproduce the problem:

• make sure version 3.5(.3) is specified in tox.ini, Makefile and setup.py
• make clean && make tox

you will get below error.

It appears that either setup.py can't find README.md (incorrect path?) or open() method cannot work for other reason. Maybe invocation of open() must be done in different manner for Python 3.5 version?

Would be good to have support in tox for 3.5.3 in case this package is installed in Debian 9 Stretch distribution.

GLOB sdist-make: /Users/kamil/git/get-fx/setup.py
py35 inst-nodeps: /Users/kamil/git/get-fx/.tox/.tmp/package/1/getfx-0.1.0.zip
ERROR: invocation failed (exit code 1), logfile: /Users/kamil/git/get-fx/.tox/py35/log/py35-11.log
===================================================== log start ======================================================
DEPRECATION: Python 3.5 reached the end of its life on September 13th, 2020. Please upgrade your Python as Python 3.5 is no longer maintained. pip 21.0 will drop support for Python 3.5 in January 2021. pip 21.0 will remove support for this functionality.
Processing ./.tox/.tmp/package/1/getfx-0.1.0.zip
    ERROR: Command errored out with exit status 1:
     command: /Users/kamil/git/get-fx/.tox/py35/bin/python -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/var/folders/1_/gkm6s3xn2kxgc4xjhvyj8rb00000gn/T/pip-req-build-ztwgck2g/setup.py'"'"'; __file__='"'"'/private/var/folders/1_/gkm6s3xn2kxgc4xjhvyj8rb00000gn/T/pip-req-build-ztwgck2g/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /private/var/folders/1_/gkm6s3xn2kxgc4xjhvyj8rb00000gn/T/pip-pip-egg-info-0n6484og
         cwd: /private/var/folders/1_/gkm6s3xn2kxgc4xjhvyj8rb00000gn/T/pip-req-build-ztwgck2g/
    Complete output (5 lines):
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/private/var/folders/1_/gkm6s3xn2kxgc4xjhvyj8rb00000gn/T/pip-req-build-ztwgck2g/setup.py", line 6, in <module>
        with open("README.md", "r", encoding="utf-8") as fh:
    FileNotFoundError: [Errno 2] No such file or directory: 'README.md'
    ----------------------------------------
WARNING: Discarding file:///Users/kamil/git/get-fx/.tox/.tmp/package/1/getfx-0.1.0.zip. Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.