BaseOutlineCompiler::importTTX: sfntVersion scrubbing fails if TTX file has CRLF line endings

1. Convert `tests\data\TestFont.ufo\data\com.github.fonttools.ttx\CUST.ttx` to CRLF line endings
2. Run `test_importTTX`
```
________________ OutlineOTFCompilerTest.test_importTTX[defcon] ________________

self =
testufo =

def test_importTTX(self, testufo):
compiler = OutlineOTFCompiler(testufo)
otf = compiler.otf = TTFont(sfntVersion="OTTO")
compiler.importTTX()
assert "CUST" in otf
assert otf["CUST"].data == b"\x00\x01\xbe\xef"
> assert otf.sfntVersion == "OTTO"
E AssertionError: assert '\x00\x01\x00\x00' == 'OTTO'
E - ☺
E + OTTO

tests\outlineCompiler_test.py:446: AssertionError
_______________ OutlineOTFCompilerTest.test_importTTX[ufoLib2] ________________

self =
testufo =

def test_importTTX(self, testufo):
compiler = OutlineOTFCompiler(testufo)
otf = compiler.otf = TTFont(sfntVersion="OTTO")
compiler.importTTX()
assert "CUST" in otf
assert otf["CUST"].data == b"\x00\x01\xbe\xef"
> assert otf.sfntVersion == "OTTO"
E AssertionError: assert '\x00\x01\x00\x00' == 'OTTO'
E - ☺
E + OTTO

tests\outlineCompiler_test.py:446: AssertionError
```
I asked in #python and was told that `re.MULTILINE` does not consider `\r\n`. Seeing as this is hack at best, should this maybe be done in fontTools instead, i.e. as a kwarg saying that `sfntVersion` should not be imported? Or make the import function accept a XML object so we can do whatever we want beforehand?

Reproduce the failure by converting tests/data/TestFont.ufo/data/com.github.fonttools.ttx/CUST.ttx to CRLF line endings and running test_importTTX in tests/outlineCompiler_test.py. Examine BaseOutlineCompiler::importTTX and the reported sfntVersion assertion; done should preserve the CUST data while keeping sfntVersion equal to "OTTO".

Written by the indexing model from the issue text.