chore(workaround,test): pin coverage to 7.6.3

Underlying issue is with 7.6.5: reported to
nedbat/coveragepy#1891 . First 7.6.1 was tried
and it worked. Next bisection is 7.6.3. We will stop on highest working.

setup.py

@@ -87,7 +87,7 @@
     ],
     'devel-utils': [
         'asv',        # benchmarks
-        'coverage',
+        'coverage==7.6.3',
         'gprof2dot',  # rendering cProfile output as a graph image
         'psutil',
         'pytest-xdist',  # parallelize pytest runs etc