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='2020-05-04 09:56:12 +0200'>2020-05-04</span></td><td><a href='/guix/commit/tests/rsa.key?id=43408e304ffb1149f35cb539b40d673d567c9116'>Add (guix openpgp).</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/openpgp.scm, tests/openpgp.scm, tests/civodul.key,
tests/dsa.key, tests/ed25519.key, tests/rsa.key,
tests/ed25519.sec: New files.
* Makefile.am (MODULES): Add guix/openpgp.scm.
(SCM_TESTS): Add tests/openpgp.scm.
(EXTRA_DIST): Add tests/*.key and tests/ed25519.sec.
</span></span></td><td>Ludovic Courtès</td></tr>