Fix too low cpu_times_percent values when interval < ~1 second (#1586)

[frankkusters]

Summary

• OS: n/a
• Bug fix: yes
• Type: core
• Fixes: [Ubuntu 16.04.6 LTS] cpu_times_percent wrong idle values when percpu=True #1586

Description

Implement suggestion by @ulxasc in #1586 (comment) . This fixes that cpu_times_percent() reports much too low values if the interval is less than 1 second (with percpu=True) or less than 1/cpu_count() seconds (with percpu=False).

[giampaolo]

With this change I get one failure:

======================================================================
FAIL: psutil.tests.test_linux.TestMisc.test_cpu_steal_decrease
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Traceback (most recent call last):
  File "/home/giampaolo/svn/psutil/psutil/tests/test_linux.py", line 1569, in test_cpu_steal_decrease
    self.assertNotEqual(sum(cpu_times_percent), 100.0)
AssertionError: 100.0 == 100.0