Since wheel support was introduced in 2015 we always relied on pre-built
wheels for bootstrapping. Now, we can bootstrap directly from the
sources of these packages in git.
The `bootstrapped-pip` packages is used to build `pip`, `setuptools` and `wheel`,
after which those packages are used to build everything else.
Note that when building `bootstrapped-pip` some errors are shown.
These are not important, the build actually does succeed and work as intended.
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