Serf 1.3.9 relies on presence of "python" executable

[Christoph Conrads <ch...@inria.fr>]

To whom it may concern,

Serf 1.3.9 cannot be built on Devuan Beowulf (like Debian 10) because the
build relies on the presence of an executable called "python". This
executable cannot be relied on anymore to exist.

You can see the error message and the rationale for why this executable
is unlikely to exist in Spack issue 25408:
https://github.com/spack/spack/issues/25408

The issue is independent of Spack; there are at least four Python files
that rely on the presence of the "python" executable:
* build/check.py
* build/gen_def.py
* test/certs/create_certs.py
* buckets/hpack_huffman.py

Christoph Conrads