This bug haunts me on one single box and I can not fathom what the cause might be. Almost every port has been recompiled by now. The base system ist running CURRENT (13.0-CURRENT #30 r356540: Thu Jan  9 06:11:52 UTC 2020).

The transition from Python 3.6 to 3.7 has already been performed. Any ideas?

[...]
===>  Configuring for py37-sphinx-1.6.5_2,1
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "setup.py", line 91, in <module>
    from babel.messages.pofile import read_po
  File "/usr/local/lib/python3.7/site-packages/babel/messages/__init__.py", line 12, in <module>
    from babel.messages.catalog import *
  File "/usr/local/lib/python3.7/site-packages/babel/messages/catalog.py", line 24, in <module>
    from babel.dates import format_datetime
  File "/usr/local/lib/python3.7/site-packages/babel/dates.py", line 29, in <module>
    from babel.util import UTC, LOCALTZ
  File "/usr/local/lib/python3.7/site-packages/babel/util.py", line 20, in <module>
    from babel import localtime
  File "/usr/local/lib/python3.7/site-packages/babel/localtime/__init__.py", line 74, in <module>
    LOCALTZ = get_localzone()
  File "/usr/local/lib/python3.7/site-packages/babel/localtime/__init__.py", line 70, in get_localzone
    return _get_localzone()
  File "/usr/local/lib/python3.7/site-packages/babel/localtime/_unix.py", line 126, in _get_localzone
    return pytz.tzfile.build_tzinfo('local', tzfile)
  File "/usr/local/lib/python3.7/site-packages/pytz/tzfile.py", line 29, in build_tzinfo
    typecnt, charcnt) = unpack(head_fmt, fp.read(head_size))
struct.error: unpack requires a buffer of 44 bytes
*** Error code 1

Stop.
make: stopped in /usr/ports/textproc/py-sphinx