bugfix for make_test_data which failed with numpy < 1.17

[venaturum]

• [x] closes #80
• [ ] tests added / passed
• [x] ensure all linting tests pass
• [x] changelog entry

[codecov-commenter]

Codecov Report

Base: 96.13% // Head: 96.04% // Decreases project coverage by -0.09% :warning:

Coverage data is based on head (ff1f4ac) compared to base (2b4b059). Patch coverage: 85.71% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #156 +/- ## ========================================== - Coverage 96.13% 96.04% -0.10% ========================================== Files 33 33 Lines 1990 1995 +5 ========================================== + Hits 1913 1916 +3 - Misses 77 79 +2 ``` | [Impacted Files](https://codecov.io/gh/staircase-dev/staircase/pull/156?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [staircase/test\_data.py](https://codecov.io/gh/staircase-dev/staircase/pull/156/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3RhaXJjYXNlL3Rlc3RfZGF0YS5weQ==) | `95.45% <85.71%> (-4.55%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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