Starting in Python 3.7 the quoting support changed from RFC 2396 to RFC 3986.

- Expansion of a list including `None` values, violating 3.2.1 as the
  undefined value is not ignored, e.g.

  >>> expand('/test{/x*}', x=['one', None, 'three'])
  '/test/one/None/three'

- A list with empty string was ignored, violating 2.3 as this should not
  be tread as undefined, example:

  >>> expand('/test{/x}', x=[])
  '/test'
  >>> expand('/test{/x}', x='')
  '/test/'
  >>> expand('/test{/x}', x=[''])
  '/test'  # should be /test/
  >>> expand('/test{/x}', x=['', ''])
  '/test/,'
  >>> expand('/test{/x*}', x=['', ''])
  '/test//'
  >>> expand('/test{/x*}', x=[''])
  '/test'

At some point the uritemplate module added a variables function. This
adds that interface to maintain compatibility.

Closes #26