flake8 requirements: Update pyparsing to 2.1.6

- *Major packrat upgrade*, inspired by patch provided by Tal Einat -
  many, many, thanks to Tal for working on this! Tal's tests show
  faster parsing performance (2X in some tests), *and* memory reduction
  from 3GB down to ~100MB! Requires no changes to existing code using
  packratting. (Uses OrderedDict, available in Python 2.7 and later.
  For Python 2.6 users, will attempt to import from ordereddict
  backport. If not present, will implement pure-Python Fifo dict.)

- Minor API change - to better distinguish between the flexible
  numeric types defined in pyparsing_common, I've changed "numeric"
  (which parsed numbers of different types and returned int for ints,
  float for floats, etc.) and "number" (which parsed numbers of int
  or float type, and returned all floats) to "number" and "fnumber"
  respectively. I hope the "f" prefix of "fnumber" will be a better
  indicator of its internal conversion of parsed values to floats,
  while the generic "number" is similar to the flexible number syntax
  in other languages. Also fixed a bug in pyparsing_common.numeric
  (now renamed to pyparsing_common.number), integers were parsed and
  returned as floats instead of being retained as ints.

- Fixed bug in upcaseTokens and downcaseTokens introduced in 2.1.5,
  when the parse action was used in conjunction with results names.
  Reported by Steven Arcangeli from the dql project, thanks for your
  patience, Steven!

- Major change to docs! After seeing some comments on reddit about
  general issue with docs of Python modules, and thinking that I'm a
  little overdue in doing some doc tuneup on pyparsing, I decided to
  following the suggestions of the redditor and add more inline examples
  to the pyparsing reference documentation. I hope this addition
  will clarify some of the more common questions people have, especially
  when first starting with pyparsing/Python.

- Deprecated ParseResults.asXML. I've never been too happy with this
  method, and it usually forces some unnatural code in the parsers in
  order to get decent tag names. The amount of guesswork that asXML
  has to do to try to match names with values should have been a red
  flag from day one. If you are using asXML, you will need to implement
  your own ParseResults->XML serialization. Or consider migrating to
  a more current format such as JSON (which is very easy to do:
  results_as_json = json.dumps(parse_result.asDict()) Hopefully, when
  I remove this code in a future version, I'll also be able to simplify
  some of the craziness in ParseResults, which IIRC was only there to try
  to make asXML work.

- Updated traceParseAction parse action decorator to show the repr
  of the input and output tokens, instead of the str format, since
  str has been simplified to just show the token list content.

  (The change to ParseResults.__str__ occurred in pyparsing 2.0.4, but
  it seems that didn't make it into the release notes - sorry! Too
  many users, especially beginners, were confused by the
  "([token_list], {names_dict})" str format for ParseResults, thinking
  they were getting a tuple containing a list and a dict. The full form
  can be seen if using repr().)

  For tracing tokens in and out of parse actions, the more complete
  repr form provides important information when debugging parse actions.

misc/requirements/requirements-flake8.txt

@@ -21,5 +21,5 @@ pep8-naming==0.4.1
 pycodestyle==2.0.0
 pydocstyle==1.0.0
 pyflakes==1.2.3
-pyparsing==2.1.5
+pyparsing==2.1.6
 six==1.10.0