Update call and expected returns from pandas's parse_time_string

The API for `parse_time_string` changed recently to only return two values
instead of three [1] and the function is no longer importable from the pandas
core tools datetimes module as of version 1.1.0. Instead, we have to import the
`parsing` module that exposes the `parse_time_string` function.

This bug should affect anyone trying to run augur with pandas 1.1.0 but not
users running earlier versions of pandas.

[1] pandas-dev/pandas#31065

augur/parse.py

@@ -25,7 +25,8 @@ def fix_dates(d, dayfirst=True):
     On failure to parse the date, the function will return the input.
     '''
     try:
-        dto, _, res = pd.core.tools.datetimes.parse_time_string(d, dayfirst=dayfirst)
+        from pandas.core.tools.datetimes import parsing
+        dto, res = parsing.parse_time_string(d, dayfirst=dayfirst)
         if res == 'year':
             return "%d-XX-XX"%dto.year
         elif res == 'month':