Bump astropy from 4.3.post1 to 5.3.3 in /docs (#29)

Bumps [astropy](https://github.com/astropy/astropy) from 4.3.post1 to 5.3.3.
- [Release notes](https://github.com/astropy/astropy/releases)
- [Changelog](https://github.com/astropy/astropy/blob/main/docs/changelog.rst)
- [Commits](https://github.com/astropy/astropy/commits/v5.3.3)

---
updated-dependencies:
- dependency-name: astropy
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/requirements.txt

@@ -1,4 +1,4 @@
-astropy==4.3.post1
+astropy==5.3.3
 attrs==21.2.0
 cloudpickle==1.6.0
 dask==2021.7.2