vimoption2python no longer creates ambiguous regex syntax

[Hamled]

Python 3.7+ makes it so that a FutureWarning notice is printed whenever a regular expression is compiled that contains a sequence which might become ambiguous if nested set operations are added.

There are some cases where Deoplete's vimoption2python function could produce such a sequence when generating a Python regex for the plugin code to use. In particular, I found this happened from the built-in scheme filetype.

The iskeyword setting for Scheme is 33,35-39,42-43,45-58,60-90,94,95,97-122,126 which is converted by vimoptions2python into [\w!#-'*-+--:<-Z^_a-z~] (the issue is the 4th pattern which gets converted into --:).

The change in this PR inserts an additional check before adding a pattern to the list, to determine if it's a character range and either the beginning or end character in the range is -. If that is the case, we escape the literal hyphen (e.g. if the pattern was !-- we would escape it to !-\- instead of !\--). I don't believe we will encounter the other ambiguous regex syntax cases in this code path.

All of that being said, I'm very unfamiliar with Vimscript so I would encourage a thorough review and consideration of any possible edge cases I've missed!

[codecov-io]

Codecov Report

Merging #939 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #939   +/-   ##
=======================================
  Coverage   47.88%   47.88%           
=======================================
  Files          18       18           
  Lines         875      875           
=======================================
  Hits          419      419           
  Misses        456      456