Issue with case sensitivity on MacOS X

Astropy has ``M_bol`` and ``m_bol`` objects in the same sub-package, and this causes issues when using sphinx-automodapi because MacOS X is case-insensitive meaning that the .rst pages for each of these objects will conflict:

```
/Users/tom/Dropbox/Code/Astropy/astropy/docs/index.rst:: WARNING: toctree contains reference to nonexisting document 'api/astropy.units.function.logarithmic.m_bol'
/Users/tom/Dropbox/Code/Astropy/astropy/docs/index.rst:: WARNING: toctree contains reference to nonexisting document 'api/astropy.units.function.logarithmic.m_bol'
/Users/tom/Dropbox/Code/Astropy/astropy/docs/units/index.rst:389::1: WARNING: py:obj reference target not found: astropy.units.function.logarithmic.m_bol
/Users/tom/Dropbox/Code/Astropy/astropy/docs/units/index.rst:: WARNING: toctree contains reference to nonexisting document 'api/astropy.units.function.logarithmic.m_bol'
/Users/tom/Dropbox/Code/Astropy/astropy/docs/units/index.rst:389::: WARNING: toctree contains reference to nonexisting document 'api/astropy.units.function.logarithmic.m_bol'
/Users/tom/Dropbox/Code/Astropy/astropy/docs/units/logarithmic_units.rst:244::1: WARNING: py:obj reference target not found: astropy.units.function.logarithmic.m_bol
/Users/tom/Dropbox/Code/Astropy/astropy/docs/units/logarithmic_units.rst:244::: WARNING: toctree contains reference to nonexisting document 'api/astropy.units.function.logarithmic.m_bol'
```

Start by reproducing the Sphinx documentation build on macOS and inspect the conflicting API pages generated for M_bol and m_bol. Check docs/index.rst, docs/units/index.rst, and docs/units/logarithmic_units.rst; done means both objects have distinct generated documentation pages without the reported missing-document or reference warnings.

Written by the indexing model from the issue text.