Signed-off-by: Dan Callahan <danc@element.io>

In #75, bytecode was disabled (from a bit of FUD back in `python<2.4` days, according to dev chat), I think it's safe enough to enable it again.

Added in `__pycache__/` and `.pyc`/`.pyd` to `.gitignore`, to extra-insure compiled files don't get committed.

`Signed-off-by: Jonathan de Jong <jonathan@automatia.nl&gt;`>

Adds note about updating dh-virtualenv once we drop support for Xenial.

We can't update now, because it needs debhelper 12, while Xenial only
backports 10.

Signed-off-by: Dan Callahan <danc@element.io>

`distutils` is pretty much deprecated these days, and replaced with
`setuptools`. It's also annoying because it's you can't `pip install` it, and
it's hard to figure out which debian package we should depend on to make sure
it's there.

Since we only use it for a tiny function anyway, let's just vendor said
function into our codebase.

Debian package builds were failing for two reasons:

 1. Python versions prior to 3.7 throw exceptions when attempting to print
    Unicode characters under a "C" locale. (#9076)

 2. We depended on `dh-systemd` which no longer exists in Debian Bullseye, but
    is necessary in Ubuntu Xenial. (#9073)

Setting `LANG="C.UTF-8"` in the build environment fixes the first issue.
See also: https://bugs.python.org/issue19846



The second issue is a bit trickier. The dh-systemd package was merged into
debhelper version 9.20160709 and a transitional package left in its wake.

The transitional dh-systemd package was removed in Debian Bullseye.

However, Ubuntu Xenial ships an older debhelper, and still needs dh-systemd.

Thus, builds were failing on Bullseye since we depended on a package which had
ceased existing, but we couldn't remove it from the debian/control file and our
build scripts because we still needed it for Ubuntu Xenial.

We can fix the debian/control issue by listing dh-systemd as an alternative to
the newer versions of debhelper. Since dh-systemd declares that it depends on
debhelper, Ubuntu Xenial will select its older dh-systemd which will in turn
pull in its older debhelper, resulting in no change from the status quo. All
other supported releases will satisfy the debhelper dependency constraint and
skip the dh-systemd alternative.

Build scripts were fixed by unconditionally attempting to install dh-systemd on
all releases and suppressing failures.

Once we drop support for Ubuntu Xenial, we can revert most of this commit and
rely on the version constraint on debhelper in debian/control.

Fixes #9076
Fixes #9073

Signed-off-by: Dan Callahan <danc@element.io>

After https://github.com/matrix-org/synapse/pull/8377, the deb packages no longer indirectly installed the `"test"` dependencies, causing debian packages to fail to build while carrying out the unit tests.

This PR installs `test` dependencies explicitly when building debian packages.