log which python version to use/was used

eg add this to the header:

```
 # This file is autogenerated by pip-compile on:
 # Python 2.7.15+ (default, Oct  2 2018, 22:12:08) 
 # [GCC 8.2.0] on linux2
 # To update, run:
 #
 #    pip-compile --generate-hashes --output-file=requirements.txt requirements.in
```