[pip] move the external requirement into the setup

[das7pad]

It is possible to install aiosocks with an aiohttp version below 2.3.2. This should not be possible and pip can help us here.

See this demo using a new virtual environment running in /tmp/001:

$ python3 -m venv /tmp/001 && /tmp/001/bin/pip install -q aiohttp==2.3.0 && \
  /tmp/001/bin/pip install -q aiosocks && \
  /tmp/001/bin/python3 -c "import aiosocks.connector"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/tmp/001/lib/python3.5/site-packages/aiosocks/connector.py", line 17, in <module>
    raise RuntimeError('aiosocks.connector depends on aiohttp 2.3.2+')
RuntimeError: aiosocks.connector depends on aiohttp 2.3.2+
$ echo $?
1

$ rm -rf /tmp/001/

$ python3 -m venv /tmp/001 && /tmp/001/bin/pip install -q aiohttp==2.3.0 && \
  /tmp/001/bin/pip install -q git+https://github.com/das7pad/aiosocks@dbbc && \
  /tmp/001/bin/python3 -c "import aiosocks.connector"
$ echo $?
0

Packages using pip-compile or similar rely on a version specifier of requirements to choose a working version mix of external packages.

[coveralls]

Coverage increased (+0.2%) to 96.438% when pulling dbbc993 on das7pad:move-requirement into 932374c on nibrag:master.

[nibrag]

#37