Lock evdev dependency for Python 2.

[bcipriano]

Installing dependencies is currently failing for Python 2:

Collecting evdev>=1.3; "linux" in sys_platform
  Downloading evdev-1.6.1.tar.gz (26 kB)
    ERROR: Command errored out with exit status 1:
     command: /usr/local/bin/python -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-6TVjAa/evdev/setup.py'"'"'; __file__='"'"'/tmp/pip-install-6TVjAa/evdev/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-pip-egg-info-bH2XrD
         cwd: /tmp/pip-install-6TVjAa/evdev/
    Complete output (6 lines):
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-6TVjAa/evdev/setup.py", line 103
        cmd = [sys.executable, 'evdev/genecodes.py', *headers]

The evdev is used by our recently added pynput dependency, but only on Linux.

The evdev project dropped Python 2 support in v1.5.0, however it continued to work until now. The recent v1.6.1 release uses a syntax that is no longer compatible with Python 2.

On Linux, lock evdev to 1.4.0, the last version to support Python 2. This still satisfies pynput's requirement of evdev >= 1.3.

[bcipriano]

Merging to unbreak Python 2 tests in our pipelines.