Documentation issue: Compiling Bazel from source

Documentation URL: https://docs.bazel.build/versions/master/install-compile-source.html

> Python. Versions 2 and 3 are supported, installing one of them is enough.

But compiling Bazel 3.1.0 with Python 2 fails with the following error.

```
ERROR: /[...]/src/BUILD:305:2: Executing genrule //src:embedded_tools_nojdk failed (Exit 1): bash failed: error executing command
(cd /tmp/bazel_E1NyC7jk/out/execroot/io_bazel && \
exec env - \
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin \
/bin/bash -c 'source external/bazel_tools/tools/genrule/genrule-setup.sh; bazel-out/host/bin/src/create_embedded_tools "bazel-out/k8-opt/bin/src/embedded_tools_nojdk.zip" bazel-out/k8-opt/bin/src/embedded_tools_nojdk.params')
Execution platform: //:default_host_platform
Error occurred while attempting to use the default Python toolchain (@rules_python//python:autodetecting_toolchain).
According to '/usr/bin/python -V', version is 'Python 2.7.5', but we need version 3. PATH is:

[...]
```

Open the documentation at https://docs.bazel.build/versions/master/install-compile-source.html and compare its Python version guidance with the Bazel 3.1.0 failure reported in the issue. Confirm which Python version the source build requires, then update the compilation instructions so the documented requirement matches the observed behavior.

Written by the indexing model from the issue text.