MAINT: drop pinned sphinx-gallerty version

[CHANGED] switch to using nbsphinx for the gallery eliminated
    specific version requirement. Note that we still use sphinx-gallery
    for its CSS style functions

requirements.txt

@@ -12,7 +12,7 @@ nbsphinx
 nbformat
 nbconvert>5.4
 sphinxcontrib-bibtex
-sphinx-gallery==0.7
+sphinx-gallery
 sphinx_bootstrap_theme
 sphinx_panels
 git+https://github.com/cogent3/cogent3.git@develop#egg=cogent3[dev]