Error encountered with Eventbrite "Webhook Tutorial Using Flask"

Output of "pip3 install -r requirements.txt"

(vpy3) pete@bgs:~/Dropbox/shared/python/flask/everbrite$ pip install -r requirements.txt
Collecting Flask==0.10.1 (from -r requirements.txt (line 2))
  Downloading Flask-0.10.1.tar.gz (544kB)
    100% |████████████████████████████████| 552kB 1.2MB/s 
Collecting Jinja2==2.7.3 (from -r requirements.txt (line 3))
  Downloading Jinja2-2.7.3.tar.gz (378kB)
    100% |████████████████████████████████| 378kB 1.2MB/s 
Requirement already satisfied (use --upgrade to upgrade): MarkupSafe==0.23 in ./vpy3/lib/python3.5/site-packages (from -r requirements.txt (line 4))
Exception:
Traceback (most recent call last):
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2711, in _dep_map
    return self.__dep_map
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2545, in __getattr__
    raise AttributeError(attr)
AttributeError: _DistInfoDistribution__dep_map

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2702, in _parsed_pkg_info
    return self._pkg_info
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2545, in __getattr__
    raise AttributeError(attr)
AttributeError: _pkg_info

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/basecommand.py", line 215, in main
    status = self.run(options, args)
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/commands/install.py", line 310, in run
    wb.build(autobuilding=True)
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/wheel.py", line 750, in build
    self.requirement_set.prepare_files(self.finder)
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/req/req_set.py", line 370, in prepare_files
    ignore_dependencies=self.ignore_dependencies))
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/req/req_set.py", line 657, in _prepare_file
    set(req_to_install.extras) - set(dist.extras)
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2670, in extras
    return [dep for dep in self._dep_map if dep]
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2713, in _dep_map
    self.__dep_map = self._compute_dependencies()
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2722, in _compute_dependencies
    for req in self._parsed_pkg_info.get_all('Requires-Dist') or []:
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 2704, in _parsed_pkg_info
    metadata = self.get_metadata(self.PKG_INFO)
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 1474, in get_metadata
    return self._get(self._fn(self.egg_info, name)).decode("utf-8")
  File "/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/pip/_vendor/pkg_resources/__init__.py", line 1580, in _get
    with open(path, 'rb') as stream:
FileNotFoundError: [Errno 2] No such file or directory: '/home/pete/Dropbox/shared/python/flask/everbrite/vpy3/lib/python3.5/site-packages/markupsafe-0.23.dist-info/METADATA'
(vpy3) pete@bgs:~/Dropbox/shared/python/flask/everbrite$