Disallow function redefinitions (E0102).
Unshadow function definitions.

* Silence pylint warnings on behave tests. Naming all steps
  as `step_impl` is conventional.
Merge pull request #709 from Renelvon/no_redefinition

Disallow function redefinition (E0102).
Flag implicit relative imports (W0403).
State relative imports explicitly.

* Reorder imports in accordance to PEP8.
* Replace calls to privtopub with calls to
  privkey_to_pubkey for consistency.