PodGen installation on TravisCI fails for Python 3.7

[tobinus]

The weekly build on TravisCI has been failing for some time now on Python 3.7: https://app.travis-ci.com/github/tobinus/python-podgen/builds/248003090

The relevant part of the build log:

pip install -r requirements.txt
Obtaining file:///home/travis/build/tobinus/python-podgen (from -r requirements.txt (line 3))
  Preparing metadata (setup.py) ... error
  ERROR: Command errored out with exit status 1:
   command: /home/travis/virtualenv/python3.7.1/bin/python -c 'import io, os, sys, setuptools, tokenize; sys.argv[0] = '"'"'/home/travis/build/tobinus/python-podgen/setup.py'"'"'; __file__='"'"'/home/travis/build/tobinus/python-podgen/setup.py'"'"';f = getattr(tokenize, '"'"'open'"'"', open)(__file__) if os.path.exists(__file__) else io.StringIO('"'"'from setuptools import setup; setup()'"'"');code = f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-1clsu09f
       cwd: /home/travis/build/tobinus/python-podgen/
  Complete output (21 lines):
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/setuptools/__init__.py", line 18, in <module>
      from setuptools.dist import Distribution
    File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/setuptools/dist.py", line 34, in <module>
      from ._importlib import metadata
    File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/setuptools/_importlib.py", line 28, in <module>
      disable_importlib_metadata_finder(metadata)
    File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/setuptools/_importlib.py", line 12, in disable_importlib_metadata_finder
      import importlib_metadata
    File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/importlib_metadata/__init__.py", line 471, in <module>
      __version__ = version(__name__)
    File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/importlib_metadata/__init__.py", line 438, in version
      return distribution(package).version
    File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/importlib_metadata/__init__.py", line 411, in distribution
      return Distribution.from_name(package)
    File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/importlib_metadata/__init__.py", line 179, in from_name
      dists = resolver(name)
    File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/setuptools/_vendor/importlib_metadata/__init__.py", line 886, in find_distributions
      found = self._search_paths(context.name, context.path)
  AttributeError: 'str' object has no attribute 'name'

My first guess is that this is a bug in TravisCI's environment, and the findings in pypa/setuptools#3118 support this.

Instead of screwing around with TravisCI, I'd rather switch to using GitHub Actions for CI/CD, though I guess a simple fix is to pin the setuptools version.