* Review F401,F841,F842 flake8 errors (unused variables, imports)

* Review comments

* More tests in test_installed_npymath_ini

* Review comments

TST: test dims match on lstsq()

* tensorinv() was previously completely
uncovered by unit tests

* replace most usage of SkipTest() with
pytest.skip()

* where possible, we avoid use of the standard
library SkipTest because unittest skipping
is routed through the pytest nose compatibility
layer in that scenario, which can prevent an easy
trace back to the test line where the skip
occurred

When code does:
```python
def test():
    def check(dtype):
        # code
    for dtype in [types]:
        check(dtype)
```
replace it with:
```python
@pytest.mark.parametrize('dtype', [types])
def test(dtype):
    # code
```

BUG: Make matrix_power again work for object arrays.

This fixes a regression introduced in #10985. Using matmul instead of
dot lost the object type while adding the ability to deal with matrix
stacks. This implements a partial fix by using dot for 2-D object
arrays, but object array stacks cannot be handled.

Closes #11635.