Fix error when key contains escape sequences

[edwintorok]

Python's regex engine tries to interpet escape sequences inside the replacement string, and throws an exception if it finds an invalid one. However the replacement is meant to be a literal string here, we do not want to process anything that would look like an escape sequence.

Fixes this error on startup:

Error detected while processing function zotcite#Init[24]..zotcite#GlobalInit[17]..provider#python3#Call:
line   18:
Traceback (most recent call last):
  File "/usr/lib/python3.7/sre_parse.py", line 1015, in parse_template
    this = chr(ESCAPES[this][1])
KeyError: '\\s'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/edwin/.vim/pack/my-plugins/start/zotcite/python3/zotero.py", line 265, in __init__
    self._load_zotero_data()
  File "/home/edwin/.vim/pack/my-plugins/start/zotcite/python3/zotero.py", line 313, in _load_zotero_data
    self._calculate_citekeys()
  File "/home/edwin/.vim/pack/my-plugins/start/zotcite/python3/zotero.py", line 469, in _calculate_citekeys
    key = re.sub('{authors}', lastnames.lower(), key)
  File "/usr/lib/python3.7/re.py", line 192, in sub
    return _compile(pattern, flags).sub(repl, string, count)
  File "/usr/lib/python3.7/re.py", line 309, in _subx
    template = _compile_repl(template, pattern)
  File "/usr/lib/python3.7/re.py", line 300, in _compile_repl
    return sre_parse.parse_template(repl, pattern)
  File "/usr/lib/python3.7/sre_parse.py", line 1018, in parse_template
    raise s.error('bad escape %s' % this, len(this))
re.error: bad escape \s at position

With this change zotcite becomes usable, and things like <leader>zi and <leader>zo produce plausible results with author names and titles. Omnicompletion shows only authornames, no titles, so there might be more bugs, but at least zotcite started working now for me.

[jalvesaq]

Thanks!