error thrown when building snapd snap

don't have time right now to debug this further today, so throwing the error up here in case someone else wants to look at it in the meantime:
```
Building fontconfig-bionic
... many lines
dpkg-buildpackage: binary and diff upload (original source NOT included)
Error in atexit._run_exitfuncs:
Traceback (most recent call last):
File "./build-from-security.py", line 53, in cleanup_self_build_fontconfig
subprocess.check_call("sudo", "apt-get", "remove", "-y", "libfreetype6-dev", cwd=tmp)
File "/usr/lib/python3.5/subprocess.py", line 576, in check_call
retcode = call(*popenargs, **kwargs)
File "/usr/lib/python3.5/subprocess.py", line 557, in call
with Popen(*popenargs, **kwargs) as p:
File "/usr/lib/python3.5/subprocess.py", line 855, in __init__
raise TypeError("bufsize must be an integer")
TypeError: bufsize must be an integer
```

notably fontconfig-bionic was built first, not sure if that is related, but full build log is here: https://pastebin.ubuntu.com/p/WzS7hT6QCm/

Start with build-from-security.py at cleanup_self_build_fontconfig, line 53, then reproduce the snap build that reaches the fontconfig-bionic step. Check the failing subprocess call against the Python 3.5 behavior shown in the traceback; done means the build cleanup completes without the TypeError.

Written by the indexing model from the issue text.