pip-compile generates setup.txt by given explicitly setup.py in a source file

[atugushev]

There is a possible wrong behavior on a command pip-compile setup.py, because it generates setup.txt file instead of requirements.txt.

Environment Versions

1. OS Type: Any (tested on MacOSX)
2. Python version: 3.7.x
3. pip version: 19.1
4. pip-tools version: 3.3.2

Steps to replicate

1. Prepare a setup.py file:

cat << EOF > setup.py
from setuptools import setup
setup(install_requires=[])
EOF

2. Run: pip-compile setup.py

Expected result

#
# This file is autogenerated by pip-compile
# To update, run:
#
#    pip-compile --output-file requirements.txt setup.py
#

Actual result

#
# This file is autogenerated by pip-compile
# To update, run:
#
#    pip-compile --output-file setup.txt setup.py
#