Python (prior to 3.8) treats meaningless string escape sequences as if
they were a slash followed by the character. That is, '\w' == '\\w'.
Python 3.8 rejects this, and it's confusing. This change fixes seven of these
regex strings found in depot_tools (through a regex search, natch). Most of
the fixes don't actually change the value of the strings, and this was
manually verified:

>>> '(/c(/.*/\+)?)?/(\d+)(/(\d+)?/?)?$' == r'(/c(/.*/\+)?)?/(\d+)(/(\d+)?/?)?$'
True
>>> '#\s*OWNERS_STATUS\s+=\s+(.+)$' == r'#\s*OWNERS_STATUS\s+=\s+(.+)$'
True
>>> 'COM\d' == r'COM\d'
True
>>> '^\s+Change-Id:\s*(\S+)$' == r'^\s+Change-Id:\s*(\S+)$'
True
>>> 'ETag:\s+([a-z0-9]{32})' == r'ETag:\s+([a-z0-9]{32})'
True

Two exceptions were the regex expressions in filter_demo_output.py and scm.py.
These were turned into raw strings despite this changing the value of the
string passed to re. This works because re supports the \x, \d, \w, \t, and
other escape sequences needed to make this work.

TL;DR - use raw strings for regex to avoid melting your brain. If bulk changing
regex strings to raw watch out for double-slashes.

Bug: 958138
Change-Id: Ic45264cfc63e8bae9cfcffe2cd88a57c2d3dcdae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1590534
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>