Drop use of the deprecated time.clock which was removed in Python 3.8.

Adapted from upstream pull request:

https://github.com/dlitz/pycrypto/pull/296

diff --git a/lib/Crypto/Random/_UserFriendlyRNG.py b/lib/Crypto/Random/_UserFriendlyRNG.py
--- a/lib/Crypto/Random/_UserFriendlyRNG.py
+++ b/lib/Crypto/Random/_UserFriendlyRNG.py
@@ -73,8 +73,11 @@ class _EntropyCollector(object):
         t = time.time()
         self._time_es.feed(struct.pack("@I", int(2**30 * (t - floor(t)))))
 
-        # Add the fractional part of time.clock()
-        t = time.clock()
+        # Add the fractional part of time.process_time()
+        try:
+           t = time.process_time()
+        except AttributeError:
+           t = time.clock()
         self._clock_es.feed(struct.pack("@I", int(2**30 * (t - floor(t)))))
 
 
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<tr><td><span title='2015-05-07 23:11:23 +0200'>2015-05-07</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=cf2abac8ec07ca421393df755211779b60739ae6'>artwork: Update to latest version.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2015-02-24 23:43:03 +0100'>2015-02-24</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=a4d8c40f7d3d5002a307b80cfe516474ea10d19f'>artwork: Update to latest GRUB image.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2015-02-12 23:43:18 +0100'>2015-02-12</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=9c09760a780d741ce39df984fc30ef728adf4d5b'>artwork: Update snapshot to GuixSD branding.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2014-11-13 23:12:46 +0100'>2014-11-13</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=c29b69be2d4633b9f8ff88b53247132337d3df47'>artwork: Update snapshot.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2014-11-12 14:28:36 +0100'>2014-11-12</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=84dfb4582121480cec7a8cfb73725ef006b747b5'>gnu: Introduce (gnu artwork) module.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>