Fix invalid escape sequences

[meator]

When trying to run

python -m compileall -q <dir>

on this project (as the XBPS package manager does), a lot of SyntaxWarning: invalid escape sequence warnings are spewed:

./future/backports/email/_header_value_parser.py:662: SyntaxWarning: invalid escape sequence '\('
  '(', '\(').replace(
./future/backports/email/_header_value_parser.py:663: SyntaxWarning: invalid escape sequence '\)'
  ')', '\)')
./future/backports/email/_header_value_parser.py:1349: SyntaxWarning: invalid escape sequence '\]'
  ''.join(ATOM_ENDS).replace('\\','\\\\').replace(']','\]'))).match
./future/backports/email/_header_value_parser.py:1352: SyntaxWarning: invalid escape sequence '\]'
  ''.join(TOKEN_ENDS).replace('\\','\\\\').replace(']','\]'))).match
./future/backports/email/_header_value_parser.py:1354: SyntaxWarning: invalid escape sequence '\]'
  ''.join(ATTRIBUTE_ENDS).replace('\\','\\\\').replace(']','\]'))).match
./future/backports/email/_header_value_parser.py:1357: SyntaxWarning: invalid escape sequence '\]'
  '\\','\\\\').replace(']','\]'))).match
./future/backports/email/_header_value_parser.py:1541: SyntaxWarning: invalid escape sequence '\ '
  """ctext = <printable ascii except \ ( )>
./future/backports/email/_header_value_parser.py:1876: SyntaxWarning: invalid escape sequence '\ '
  """ dtext = <printable ascii except \ [ ]> / obs-dtext
./future/backports/email/feedparser.py:37: SyntaxWarning: invalid escape sequence '\Z'
  NLCRE_eol = re.compile('(\r\n|\r|\n)\Z')
./future/backports/email/utils.py:68: SyntaxWarning: invalid escape sequence '\A'
  '([^\ud800-\udbff]|\A)[\udc00-\udfff]([^\udc00-\udfff]|\Z)').search
./future/backports/html/parser.py:31: SyntaxWarning: invalid escape sequence '\s'
  tagfind = re.compile('([a-zA-Z][-.a-zA-Z0-9:_]*)(?:\s|/(?!>))*')
./future/backports/html/parser.py:79: SyntaxWarning: invalid escape sequence '\s'
  endtagfind = re.compile('</\s*([a-zA-Z][-.a-zA-Z0-9:_]*)\s*>')
./future/backports/http/client.py:1: SyntaxWarning: invalid escape sequence '\_'
  """HTTP/1.1 client library
./future/backports/http/cookiejar.py:212: SyntaxWarning: invalid escape sequence '\d'
  "(\d\d\d\d) (\d\d):(\d\d):(\d\d) GMT$", re.ASCII)
./future/backports/http/cookiejar.py:293: SyntaxWarning: invalid escape sequence '\d'
  """^
./future/backports/http/cookiejar.py:429: SyntaxWarning: invalid escape sequence '\s'
  non_junk, nr_junk_chars = re.subn("^[=\s;]*", "", text)
./future/backports/test/support.py:1945: SyntaxWarning: invalid escape sequence '\d'
  m = re.match("2.6.(\d{1,2})", kernel_version)
./future/backports/urllib/parse.py:957: SyntaxWarning: invalid escape sequence '\?'
  _queryprog = re.compile('^(.*)\?([^?]*)$')

This PR fixes all of them.

[meator]

These regexes look pretty broken to be honest. All of these should be raw string literals. There are probably more examples of this in the codebase.