* fix BasicAer sampling bug (#1624)

* fix BasicAer sampling bug

* changelog

* remove legacysimulator from test_compile

Conflicts:
    CHANGELOG.rst
    test/python/tools/test_compiler.py

The changelog conflicts were inevitable as the stable branch doesn't
have the same history as master. As for the test conflict it was a
difference in the simulator (python vs c++) used, this backport
switches to use the python like on master.

(cherry picked from commit 5f7b2e7)

* Try pinning isort version

* Use constraints from last working version of lint

* Add requirements constraints too

* Bump version and prepare for 0.7.1 release

To prepare for the 0.7.1 bugfix release we need to increase the version
number and update the changelog. This commit takes care of that so we
can tag the release and build it.

…uantumRegisters and ClassicalRegisters