A hyphen breaks the devappserver

[dhermes]

Using just a regular path clean-env/ instead of the HTML-encoded clean&#2D;env/ in skip_files ends up with:

$ make language-app-run
...
INFO     2017-10-11 20:20:01,151 devappserver2.py:105] Skipping SDK update check.
INFO     2017-10-11 20:20:01,232 api_server.py:300] Starting API server at: http://localhost:41599
INFO     2017-10-11 20:20:01,349 dispatcher.py:251] Starting module "default" running at: http://localhost:8080
INFO     2017-10-11 20:20:01,351 admin_server.py:116] Starting admin server at: http://localhost:8000
Traceback (most recent call last):
  File "${HOME}/google-cloud-sdk/platform/google_appengine/_python_runtime.py", line 103, in <module>
    _run_file(__file__, globals())
  File "${HOME}/google-cloud-sdk/platform/google_appengine/_python_runtime.py", line 97, in _run_file
    execfile(_PATHS.script_file(script_name), globals_)
  File "${HOME}/google-cloud-sdk/platform/google_appengine/google/appengine/tools/devappserver2/python/runtime/runtime.py", line 192, in <module>
    main()
  File "${HOME}/google-cloud-sdk/platform/google_appengine/google/appengine/tools/devappserver2/python/runtime/runtime.py", line 169, in main
    sandbox.enable_sandbox(config)
  File "${HOME}/google-cloud-sdk/platform/google_appengine/google/appengine/tools/devappserver2/python/runtime/sandbox.py", line 344, in enable_sandbox
    from google.appengine.runtime import runtime
  File "${HOME}/google-cloud-sdk/platform/google_appengine/google/appengine/runtime/runtime.py", line 40, in <module>
    from google.appengine.runtime import cgi
  File "${HOME}/google-cloud-sdk/platform/google_appengine/google/appengine/runtime/cgi.py", line 32, in <module>
    from email import feedparser
  File "${HOME}/.pyenv/versions/2.7.14/lib/python2.7/email/feedparser.py", line 27, in <module>
    from email import message
  File "${HOME}/.pyenv/versions/2.7.14/lib/python2.7/email/message.py", line 16, in <module>
    import email.charset
  File "${HOME}/.pyenv/versions/2.7.14/lib/python2.7/email/charset.py", line 13, in <module>
    import email.base64mime
  File "${HOME}/.pyenv/versions/2.7.14/lib/python2.7/email/base64mime.py", line 40, in <module>
    from email.utils import fix_eols
  File "${HOME}/.pyenv/versions/2.7.14/lib/python2.7/email/utils.py", line 27, in <module>
    import random
  File "${HOME}/.pyenv/versions/2.7.14/lib/python2.7/random.py", line 45, in <module>
    from math import log as _log, exp as _exp, pi as _pi, e as _e, ceil as _ceil
  File "${HOME}/google-cloud-sdk/platform/google_appengine/google/appengine/tools/devappserver2/python/runtime/sandbox.py", line 1132, in load_module
    raise ImportError('No module named %s' % fullname)
ImportError: No module named math
ERROR    2017-10-11 20:20:03,377 instance.py:280] Cannot connect to the instance on localhost:17593
...

/cc @jonparrott Should I file this as a gcloud bug or GAE bug?