test_nvmet.py: test_invalid_input fails for py3

test_nvmet.py: test_invalid_input fails for py3

When you run 'make test' using python 3 the unit test
test_invalid_input fails with:

Traceback (most recent call last):
  File "/someuser/projects/nvmetcli/nvmet/test_nvmet.py", line 395, in
test_invalid_input
    for i in range(l))
  File "/someuser/projects/nvmetcli/nvmet/test_nvmet.py", line 395, in
<genexpr>
    for i in range(l))
AttributeError: module 'string' has no attribute 'lowercase'

Python 3 does not have 'string.lowercase' ref.
https://docs.python.org/3/library/string.html

Python 2 does ref.
https://docs.python.org/2/library/string.html

Both have "string.ascii_lowercase" so lets leverage that to
support both.

Signed-off-by: Tony Asleson <tasleson@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>