This is a re-opening of issue 41914 (and my first issue, so apologies for any mistakes).  Building Python 3.7, I ran into the same issue reported there (only line numbers differ):

======================================================================
FAIL: test_errors_in_command (__main__.PdbTestCase)
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Traceback (most recent call last):
  File "Lib/test/test_pdb.py", line 1535, in test_errors_in_command
    '(Pdb) ',
AssertionError: Lists differ: ['(Pd[283 chars]efined", 'LEAVING RECURSIVE DEBUGGER', '(Pdb) ', '\x1b[?1034h'] != ['(Pd[283 chars]efined", 'LEAVING RECURSIVE DEBUGGER', '(Pdb) ']

First list contains 1 additional elements.
First extra element 9:
'\x1b[?1034h'

  ['(Pdb) *** SyntaxError: unexpected EOF while parsing',
   '(Pdb) ENTERING RECURSIVE DEBUGGER',
   '*** SyntaxError: unexpected EOF while parsing',
   'LEAVING RECURSIVE DEBUGGER',
   '(Pdb) ENTERING RECURSIVE DEBUGGER',
   '> <string>(1)<module>()',
   "((Pdb)) *** NameError: name 'doesnotexist' is not defined",
   'LEAVING RECURSIVE DEBUGGER',
-  '(Pdb) ',
?          ^

+  '(Pdb) ']
?          ^

-  '\x1b[?1034h']

----------------------------------------------------------------------
Ran 39 tests in 1.024s

FAILED (failures=1)


I have also managed to determine the cause.  As can be seen, the output from this pdb invocation contains an extraneous ANSI control sequence ("interpret "meta" key, set eighth bit"). This happens when running the test inside the GNU `screen` environment.  Run the test outside `screen`, and the problem goes away.