Update pip during dev environment installation

[lagru]

Description

Running pip install -r <(cat requirements/*.txt) with pip 20.2.3 will fail with an error "Double requirement given [...]" if any requirements have appear twice with conflicting version specs. E.g. numpy at the time of this commit. Updating pip to 21.1.1 solves this issue.

See #5377 for more details.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[grlee77]

Thanks @lagru!