Fix flake8 "invalid escape sequence 'x'" warning

From, https://lintlyci.github.io/Flake8Rules/rules/W605.html

As of Python 3.6, a backslash-character pair that is not a valid
escape sequence now generates a DeprecationWarning. Although this
will eventually become a SyntaxError, that will not be for several
Python releases.

Note 'float_pobj' was simply removed from arcstat.py since it
was entirely unused.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8056

tests/test-runner/bin/zts-report.py

@@ -292,9 +292,10 @@ def process_results(pathname):
         sys.exit(1)
 
     prefix = '/zfs-tests/tests/functional/'
-    pattern = '^Test:\s*\S*%s(\S+)\s*\(run as (\S+)\)\s*\[(\S+)\]\s*\[(\S+)\]'\
+    pattern = \
+        r'^Test:\s*\S*%s(\S+)\s*\(run as (\S+)\)\s*\[(\S+)\]\s*\[(\S+)\]' \
         % prefix
-    pattern_log = '^\s*Log directory:\s*(\S*)'
+    pattern_log = r'^\s*Log directory:\s*(\S*)'
 
     d = {}
     for l in f.readlines():