Assert fails when using Z (UTC) timezone

[doekman]

WeasyPrint/weasyprint/document.py

Line 165 in 5ceddcb

assert groups['tz_hour'].startswith(('+', '-'))

Assert fails when using Z (UTC) timezone in a date in a html meta field. Stack trace:

...
  File "/Users/doeke/prj/ArchiBucket/apawa/source/apawa/utils/weasyprint.py", line 69, in render_pdf_with_overlays
    pdf = main_doc.write_pdf()
  File "/Users/doeke/prj/ArchiBucket/apawa/venv/lib/python3.7/site-packages/weasyprint/document.py", line 629, in write_pdf
    surface.set_metadata(key, _w3c_date_to_iso(value, attr))
  File "/Users/doeke/prj/ArchiBucket/apawa/venv/lib/python3.7/site-packages/weasyprint/document.py", line 165, in _w3c_date_to_iso
    assert groups['tz_hour'].startswith(('+', '-'))

This ISO date doesn't fail:

<meta name="dcterms.created" content="2019-05-09T13:34:53.564031+00:00">

This ISO date makes the assert fail (this worked before I updated weasyprint):

<meta name="dcterms.created" content="2019-05-09T13:36:37.317089Z">

The Z as timezone should be perfectly legal. Is there a reason why a custom ISO-date parser is being used?

Related: #600

[liZe]

Thank you! It's fixed now, and a test has been added to avoid regressions.