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

@@ -3,7 +3,7 @@ oemof.network==0.4.0rc0
 oemof.solph==0.4.1
 oemof.tools==0.4.0
 pandas==0.23.4
-pip==20.2.4
+pip==21.1
 Pyomo==5.7.0
 scipy==1.4.1
 xlrd==1.2.0