Add python coverage 3.7 to depot tools.

third_party/coverage/fullcoverage/encodings.py

+"""Imposter encodings module that installs a coverage-style tracer.
+
+This is NOT the encodings module; it is an imposter that sets up tracing
+instrumentation and then replaces itself with the real encodings module.
+
+If the directory that holds this file is placed first in the PYTHONPATH when
+using "coverage" to run Python's tests, then this file will become the very
+first module imported by the internals of Python 3.  It installs a
+coverage-compatible trace function that can watch Standard Library modules
+execute from the very earliest stages of Python's own boot process.  This fixes
+a problem with coverage - that it starts too late to trace the coverage of many
+of the most fundamental modules in the Standard Library.
+
+"""
+
+import sys
+
+class FullCoverageTracer(object):
+    def __init__(self):
+        # `traces` is a list of trace events.  Frames are tricky: the same
+        # frame object is used for a whole scope, with new line numbers
+        # written into it.  So in one scope, all the frame objects are the
+        # same object, and will eventually all will point to the last line
+        # executed.  So we keep the line numbers alongside the frames.
+        # The list looks like:
+        #
+        #   traces = [
+        #       ((frame, event, arg), lineno), ...
+        #       ]
+        #
+        self.traces = []
+
+    def fullcoverage_trace(self, *args):
+        frame, event, arg = args
+        self.traces.append((args, frame.f_lineno))
+        return self.fullcoverage_trace
+
+sys.settrace(FullCoverageTracer().fullcoverage_trace)
+
+# In coverage/files.py is actual_filename(), which uses glob.glob.  I don't
+# understand why, but that use of glob borks everything if fullcoverage is in
+# effect.  So here we make an ugly hail-mary pass to switch off glob.glob over
+# there.  This means when using fullcoverage, Windows path names will not be
+# their actual case.
+
+#sys.fullcoverage = True
+
+# Finally, remove our own directory from sys.path; remove ourselves from
+# sys.modules; and re-import "encodings", which will be the real package
+# this time.  Note that the delete from sys.modules dictionary has to
+# happen last, since all of the symbols in this module will become None
+# at that exact moment, including "sys".
+
+parentdir = max(filter(__file__.startswith, sys.path), key=len)
+sys.path.remove(parentdir)
+del sys.modules['encodings']
+import encodings