Breakpoint v3.0.0 is incompatible with Python libsass

[oleg-nenashev]

Follow up to sass/libsass-python#449 . 8f400aa and other commits moved the library to sass:math which is not supported in Python's libsass: https://discourse.gohugo.io/t/cant-use-sass-math-div/33087

The incompatibility leads to errors like this one:

ERROR   -  [SASS] Failed to build CSS: Error: Invalid CSS after "...   @return math": expected expression (e.g. 1px, bold), was ".div(base-conversio"
                   on line 15:19 of extra_sass/vendor/breakpoint/stylesheets/breakpoint/_helpers.scss
                   from line 28:1 of extra_sass/vendor/breakpoint/stylesheets/_breakpoint.scss
                   from line 6:4 of extra_sass/_masthead.scss
                   from line 2:1 of extra_sass/style.css.scss
           >>       @return math.div(base-conversion($value), base-conversion(breakpoint-g
              ------------------^
           Traceback (most recent call last):
             File "/home/onenashev/.local/lib/python3.8/site-packages/mkdocs_extra_sass_plugin/plugin.py", line 74, in _build_entry
               info = entry_point.save_to(site_dir, dest_dir)
             File "/home/onenashev/.local/lib/python3.8/site-packages/mkdocs_extra_sass_plugin/plugin.py", line 177, in save_to
               css, source_map = sass.compile(
             File "/home/onenashev/.local/lib/python3.8/site-packages/sass.py", line 725, in compile
               raise CompileError(v)
           sass.CompileError: Error: Invalid CSS after "...   @return math": expected expression (e.g. 1px, bold), was ".div(base-conversio"
                   on line 15:19 of extra_sass/vendor/breakpoint/stylesheets/breakpoint/_helpers.scss
                   from line 28:1 of extra_sass/vendor/breakpoint/stylesheets/_breakpoint.scss
                   from line 6:4 of extra_sass/_masthead.scss
                   from line 2:1 of extra_sass/style.css.scss
           >>       @return math.div(base-conversion($value), base-conversion(breakpoint-g

Downgrading to breakpoint 2.x works around the issue