Upgrade mkdocs to 1.3.0 to fix scripts/build failure

[florimondmanca]

Refs #2097 (comment)

CI currently broken due to mkdocs < 1.3.0 being broken after a new Jinja2 release

See: mkdocs/mkdocs#2799

+ venv/bin/mkdocs build
Traceback (most recent call last):
  File "/Users/florimond/Developer/python-projects/httpx/venv/bin/mkdocs", line 8, in <module>
    sys.exit(cli())
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.10/site-packages/click/core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.10/site-packages/click/core.py", line 1053, in main
    rv = self.invoke(ctx)
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.10/site-packages/click/core.py", line 1659, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.10/site-packages/click/core.py", line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.10/site-packages/click/core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.10/site-packages/mkdocs/__main__.py", line 187, in build_command
    build.build(config.load_config(**kwargs), dirty=not clean)
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.10/site-packages/mkdocs/config/base.py", line 216, in load_config
    from mkdocs.config.defaults import get_schema
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.10/site-packages/mkdocs/config/defaults.py", line 1, in <module>
    from mkdocs.config import config_options
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.10/site-packages/mkdocs/config/config_options.py", line 8, in <module>
    from mkdocs import utils, theme, plugins
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.10/site-packages/mkdocs/theme.py", line 6, in <module>
    from mkdocs.utils import filters
  File "/Users/florimond/Developer/python-projects/httpx/venv/lib/python3.10/site-packages/mkdocs/utils/filters.py", line 13, in <module>
    @jinja2.contextfilter
AttributeError: module 'jinja2' has no attribute 'contextfilter'

This PR upgrades to 1.3.0 which includes a fix