Fix FutureWarning for words() with "[" (#1854)

regex_opt() groups characters in sets when possible. The
warning was caused when the "[" character ended up at the beginning
of a set: r"[[...]". This emits a FutureWarning since Python 3.7
due to possible changes in semantics in the future
(https://bugs.python.org/issue30349). Just add "[" to the list
of characters that should be escaped in sets. Add unit tests
for words().

[Closes #1853.]

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