parse_segment_path: Use datetime.strptime instead of dateutil.parser

strptime is much faster but can't handle as varied formats.
But in this case we fully control the format, so there's no reason not to use it.

Profiling suggests we spend about 80% of our time in get_best_segments just parsing dates,
so this is a signifigant performance gain.

common/common/segments.py

@@ -11,8 +11,6 @@ import os
 import sys
 from collections import namedtuple
 
-import dateutil.parser
-
 from .stats import timed
 
 
@@ -58,7 +56,7 @@ def parse_segment_path(path):
 			path = path,
 			stream = stream,
 			variant = variant,
-			start = dateutil.parser.parse("{}:{}".format(hour, time)),
+			start = datetime.datetime.strptime("{}:{}".format(hour, time), "%Y-%m-%dT%H:%M:%S.%f"),
 			duration = datetime.timedelta(seconds=float(duration)),
 			is_partial = type == "partial",
 			hash = unpadded_b64_decode(hash),

common/setup.py

@@ -8,6 +8,5 @@ setup(
 		"gevent",
 		"monotonic",
 		"prometheus-client",
-		"python-dateutil",
 	],
 )