为什么travis CI中的服务器无法找到我的需求文件？

我的目录树看起来像这样

├── antimony_combinations
│   ├── antimony_combinations.py
│   ├── __init__.py
├── LICENCE
├── README.md
├── requirements.txt
├── setup.py
└── tests
    └── tests.py

我的.travis.yml样子是这样的：

language: python
before_install:    # get some dependency software. This works as expected in another package. 
  - wget https://github.com/copasi/COPASI/releases/download/Build-207/COPASI-4.25.207-Linux-64bit.tar.gz -O /tmp/copasi.tar.gz
  - tar -xzvf /tmp/copasi.tar.gz
  - export PATH=$PATH:$PWD/COPASI-4.25.207-Linux-64bit/bin/
python:
  - "3.6"
  - "3.7"
  - "3.8"
install:
  - pip install .
  - pip install -r $PWD/requirements.txt     # Also tried just using `requirements.txt` here
  - pip install nose
script:
  - nosetests
after_success:
  - coveralls

但是我得到以下错误：