Missing files in sdist

It appears that the manifest is missing at least one file necessary to build
from the sdist for version 2.0.0. You're in good company, about 5% of other
projects updated in the last year are also missing files.

```
+ /tmp/venv/bin/pip3 wheel --no-binary dm-sonnet -w /tmp/ext dm-sonnet==2.0.0
Looking in indexes: http://10.10.0.139:9191/root/pypi/+simple/
Collecting dm-sonnet==2.0.0
Downloading http://10.10.0.139:9191/root/pypi/%2Bf/065/c05b0bc40b82a/dm-sonnet-2.0.0.tar.gz (152 kB)
ERROR: Command errored out with exit status 1:
command: /tmp/venv/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-wheel-cohhce9a/dm-sonnet/setup.py'"'"'; __file__='"'"'/tmp/pip-wheel-cohhce9a/dm-sonnet/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-wheel-cohhce9a/dm-sonnet/pip-egg-info
cwd: /tmp/pip-wheel-cohhce9a/dm-sonnet/
Complete output (7 lines):
Traceback (most recent call last):
File "", line 1, in
File "/tmp/pip-wheel-cohhce9a/dm-sonnet/setup.py", line 46, in
install_requires=_parse_requirements('requirements.txt'),
File "/tmp/pip-wheel-cohhce9a/dm-sonnet/setup.py", line 22, in _parse_requirements
with open(requirements_txt_path) as fp:
FileNotFoundError: [Errno 2] No such file or directory: 'requirements.txt'
----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
```