Bug#877419: pandas: some I/O tests (hdf5, Stata) fail on non-x86

[Rebecca N. Palmer]

Re-enabling these tests found that this bug (which is probably actually 
multiple bugs) _does_ still exist, on big-endian systems and _possibly_ 
others.

pandas now (0.25.3+dfsg-7) warns the user when these are used on any 
non-x86 system.

** Stata format

- All big-endian (s390x, hppa, ppc64): 
tests/io/test_stata.py::TestStata::test_read_dta18, test_writer_117, 
test_convert_strl_name_swap, test_unicode_dta_118, test_strl_latin1 
fail.  Suggests a problem involving Unicode strings.

- ppc64el: Succeeds on the buildd, but 34 tests (mostly but not only 
involving nan/inf) fail in local qemu-ppc64el.

** HDF(5) format

- All big-endian: 
tests/io/pytables/test_pytables.py::TestHDFStore::test_legacy_table_fixed_format_read_py2, 
test_read_py2_hdf_file_in_py3, test_legacy_datetimetz_object fail. 
Suggests an issue with reading older files, possibly the same pytables 
bug (which doesn't appear to have a number) as 
https://sources.debian.org/src/pytables/3.6.1-2/debian/patches/0005-Skip-index-backcompat-tests-on-bingendian.patch/

- s390x + ppc64 but not hppa: test_flush fails.

- All big-endian: test_encoding is skipped (by upstream).

(armhf may _crash_ - see #790925 - but is not known to give wrong results.)

** Clipboard

- tests/io/test_clipboard.py::test_raw_roundtrip failed in 0.25.3+dfsg-5 
on s390x, but passed in -6 and -7 with no obvious fix in pandas. 
Possibly a bug outside pandas that has now been fixed.