Backport fix for bpo-38804

The regex http.cookiejar.LOOSE_HTTP_DATE_RE was vulnerable to regular
expression denial of service (REDoS). The regex contained multiple
overlapping \s* capture groups. A long sequence of spaces can trigger
bad performance.

See python/cpython#17157 and https://pyup.io/posts/pyup-discovers-redos-vulnerabilities-in-top-python-packages/

src/future/backports/http/cookiejar.py

@@ -225,10 +225,14 @@ def _str2time(day, mon, yr, hr, min, sec, tz):
        (?::(\d\d))?    # optional seconds
     )?                 # optional clock
        \s*
-    ([-+]?\d{2,4}|(?![APap][Mm]\b)[A-Za-z]+)? # timezone
+    (?:
+       ([-+]?\d{2,4}|(?![APap][Mm]\b)[A-Za-z]+) # timezone
+       \s*
+    )?
+    (?:
+       \(\w+\)         # ASCII representation of timezone in parens.
        \s*
-    (?:\(\w+\))?       # ASCII representation of timezone in parens.
-       \s*$""", re.X | re.ASCII)
+    )?$""", re.X | re.ASCII)
 def http2time(text):
     """Returns time in seconds since epoch of time represented by a string.
 
@@ -298,9 +302,11 @@ def http2time(text):
       (?::?(\d\d(?:\.\d*)?))?  # optional seconds (and fractional)
    )?                    # optional clock
       \s*
-   ([-+]?\d\d?:?(:?\d\d)?
-    |Z|z)?               # timezone  (Z is "zero meridian", i.e. GMT)
-      \s*$""", re.X | re. ASCII)
+   (?:
+      ([-+]?\d\d?:?(:?\d\d)?
+       |Z|z)             # timezone  (Z is "zero meridian", i.e. GMT)
+      \s*
+   )?$""", re.X | re. ASCII)
 def iso2time(text):
     """
     As for http2time, but parses the ISO 8601 formats: