gh-148192: Fix Generator._make_boundary behavior with CRLF line endings.

[henryivesjones]

The Generator._make_boundary regex does not match on boundary phrases correctly when using CRLF line endings due to re.MULTILINE not considering \r\n as a line ending.

Issue: #148192

• Issue: email.generator.Generator does not correctly identify a boundary phrase when using CRLF line endings. #148192

[python-cla-bot]

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[henryivesjones]

The failing test seems to just be flaky? I ran just that one test 4 times and 2 passed and 2 failed.

cpython % ./python.exe -m test -j 14 test_perf_profiler
Raised RLIMIT_NOFILE: 256 -> 1024
Using random seed: 1400475968
0:00:00 load avg: 4.26 Run 1 test in parallel using 1 worker process
0:00:00 load avg: 4.26 [1/1] test_perf_profiler passed

== Tests result: SUCCESS ==

1 test OK.

Total duration: 394 ms
Total tests: run=14 skipped=5
Total test files: run=1/1
Result: SUCCESS
cpython % ./python.exe -m test -j 14 test_perf_profiler
Raised RLIMIT_NOFILE: 256 -> 1024
Using random seed: 1646608690
0:00:00 load avg: 4.26 Run 1 test in parallel using 1 worker process
0:00:00 load avg: 4.26 [1/1/1] test_perf_profiler failed (1 failure)
test test_perf_profiler failed -- Traceback (most recent call last):
  File "/Users/hjones/git/forks/cpython/Lib/test/test_perf_profiler.py", line 145, in test_trampoline_works_with_forks
    self.assertEqual(process.returncode, 0)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 11 != 0


== Tests result: FAILURE ==

1 test failed:
    test_perf_profiler

Total duration: 393 ms
Total tests: run=14 failures=1 skipped=5
Total test files: run=1/1 failed=1
Result: FAILURE
cpython % ./python.exe -m test -j 14 test_perf_profiler
Raised RLIMIT_NOFILE: 256 -> 1024
Using random seed: 2888117323
0:00:00 load avg: 3.52 Run 1 test in parallel using 1 worker process
0:00:00 load avg: 3.52 [1/1/1] test_perf_profiler failed (1 failure)
test test_perf_profiler failed -- Traceback (most recent call last):
  File "/Users/hjones/git/forks/cpython/Lib/test/test_perf_profiler.py", line 145, in test_trampoline_works_with_forks
    self.assertEqual(process.returncode, 0)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 11 != 0


== Tests result: FAILURE ==

1 test failed:
    test_perf_profiler

Total duration: 388 ms
Total tests: run=14 failures=1 skipped=5
Total test files: run=1/1 failed=1
Result: FAILURE
cpython % ./python.exe -m test -j 14 test_perf_profiler
Raised RLIMIT_NOFILE: 256 -> 1024
Using random seed: 164762511
0:00:00 load avg: 3.52 Run 1 test in parallel using 1 worker process
0:00:00 load avg: 3.52 [1/1] test_perf_profiler passed

== Tests result: SUCCESS ==

1 test OK.

Total duration: 388 ms
Total tests: run=14 skipped=5
Total test files: run=1/1
Result: SUCCESS