Bump pip from 20.3.4 to 21.1 in /testsite

Bumps [pip](https://github.com/pypa/pip) from 20.3.4 to 21.1.
- [Release notes](https://github.com/pypa/pip/releases)
- [Changelog](https://github.com/pypa/pip/blob/main/NEWS.rst)
- [Commits](pypa/pip@20.3.4...21.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

testsite/requirements-legacy.txt

@@ -9,7 +9,7 @@ django-storages==1.7
 djaodjin-deployutils==0.10.1
 Jinja2==2.10.1
 Markdown==2.6.11
-pip==20.3.4                 # because deployutils missing <21.3.1
+pip==21.1                 # because deployutils missing <21.3.1
 python-dateutil==2.8.0
 requests==2.22.0
 premailer==3.6.1