Bump pip from 20.2.4 to 21.1

Bumps [pip](https://github.com/pypa/pip) from 20.2.4 to 21.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@20.2.4...21.1)

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

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -18,7 +18,7 @@ fsspec>=0.6.2
 astropy>=4.1
 gdown>=3.12.2
 ipympl>=0.7.0
-pip==20.2.4
+pip==21.1
 nbsphinx==0.8.3
 sphinx-autoapi==1.8.0
 sphinx-rtd-theme==0.5.0