Function runtest() (used by single-process mode) always returns tuple with test result as integer and test time as float.
This tuple is passed as second argument to function accumulate_result(), which has (in Python 3.6):
    def accumulate_result(self, test, result):
        ok, test_time = result
        self.test_times.append((test_time, test))

test_times is later used by code, which expects that times are floats:
        if self.ns.print_slow:
            self.test_times.sort(reverse=True)
            print("10 slowest tests:")
            for time, test in self.test_times[:10]:
                print("%s: %.1fs" % (test, time))

Code used by multi-process mode can return tuple with integer and string (describing error in subprocess), which with --slow option would cause TypeError.


Example:

In terminal 1:
$ LD_LIBRARY_PATH="$(pwd)" ./python -m test.regrtest -j2 --slow test_lib2to3

In terminal 2 (sufficiently quickly):
$ kill -SIGINT $(ps aux | grep '"test_lib2to3"' | grep -v grep | awk '{print $2}')

Output in terminal 1:

Test suite interrupted by signal SIGINT.
1 test omitted:
    test_lib2to3
10 slowest tests:
Traceback (most recent call last):
  File "/tmp/cpython/Lib/runpy.py", line 170, in _run_module_as_main
    "__main__", mod_spec)
  File "/tmp/cpython/Lib/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/tmp/cpython/Lib/test/regrtest.py", line 39, in <module>
    main_in_temp_cwd()
  File "/tmp/cpython/Lib/test/libregrtest/main.py", line 451, in main_in_temp_cwd
    main()
  File "/tmp/cpython/Lib/test/libregrtest/main.py", line 429, in main
    Regrtest().main(tests=tests, **kwargs)
  File "/tmp/cpython/Lib/test/libregrtest/main.py", line 389, in main
    self.display_result()
  File "/tmp/cpython/Lib/test/libregrtest/main.py", line 261, in display_result
    print("%s: %.1fs" % (test, time))
TypeError: a float is required


My suggested fix is to return string describing error as third element of that tuple.