Currently, core/btrfs-progs depens on python, presumably to support bindings for libbtrfsutils. However, the package doesn't contain any python-related files. This is because one needs to install the bindings with make install_python, see https://github.com/kdave/btrfs-progs/blob/master/INSTALL . On the other hand, btrfs-progs is a core package and doesn't strictly require python (the size ratio of python / btrfs-progs is ~ 26). Therefore, I suggest to move python from depends() to optdepends() (makedepends already contains python-setuptools which will pull python).

The above amount to the following change of PKGBUILD (also attached):
--- a/PKGBUILD 2018-05-07 19:38:22.191309889 -0600
+++ b/PKGBUILD 2018-05-09 19:34:32.400125421 -0600
@@ -8,8 +8,9 @@
pkgrel=2
pkgdesc='Btrfs filesystem utilities'
arch=('x86_64')
-depends=('glibc' 'libutil-linux' 'e2fsprogs' 'lzo' 'zlib' 'zstd' 'python')
+depends=('glibc' 'libutil-linux' 'e2fsprogs' 'lzo' 'zlib' 'zstd')
makedepends=('git' 'asciidoc' 'xmlto' 'systemd' 'python-setuptools')
+optdepends=('python: for libbtrfsutil python bindings')
url='https://btrfs.wiki.kernel.org'
replaces=('btrfs-progs-unstable')
conflicts=('btrfs-progs-unstable')
@@ -59,6 +60,7 @@
package() {
cd $pkgname-v$pkgver
make DESTDIR="$pkgdir" install
+ make DESTDIR="$pkgdir" install_python

# install bash completion ( FS#44618 )
install -Dm644 btrfs-completion "$pkgdir/usr/share/bash-completion/completions/btrfs"

Thanks!