Use isort directly instead of broken flake8-isort

flake8-isort appears to be broken, and unmaintained (e.g.
gforcada/flake8-isort#97).

Error, not handled by flake8 even:
https://app.circleci.com/pipelines/github/Vimjas/covimerage/5/workflows/a07d03e2-7ace-4ac7-ad02-16fdbd7508b0/jobs/2515

setup.cfg

@@ -8,7 +8,7 @@ default_section = THIRDPARTY
 force_sort_within_sections = 1
 include_trailing_comma = 1
 known_first_party = covimerage
-lines = 79
+line_length = 79
 multi_line_output = 5
 order_by_type = 1
 use_parentheses = 1

setup.py

@@ -55,7 +55,7 @@ def run(self):
 
 DEPS_QA = [
     'flake8>=3.7.0',
-    'flake8-isort',
+    'isort',
 ]
 DEPS_TESTING = [
     'pytest>=3.3.0',

tests/test_coveragepy.py

@@ -60,8 +60,10 @@ def coverage_fileobj():
 
 def test_coveragedata(coverage_fileobj):
     import coverage
+
     from covimerage.coveragepy import (
-        CoverageData, CoveragePyData, CoverageWrapperException)
+        CoverageData, CoveragePyData, CoverageWrapperException,
+    )
 
     with pytest.raises(TypeError) as excinfo:
         CoverageData(data_file='foo', cov_data=CoveragePyData())
@@ -130,8 +132,11 @@ def test_coveragedata_empty(covdata_empty):
 
 def test_coveragewrapper(coverage_fileobj, devnull):
     import coverage
+
     from covimerage.coveragepy import (
-        CoverageData, CoveragePyData, CoverageWrapper, CoverageWrapperException)
+        CoverageData, CoveragePyData, CoverageWrapper,
+        CoverageWrapperException,
+    )
 
     cov_data = CoverageWrapper()
     assert cov_data.lines == {}

tox.ini

@@ -20,6 +20,8 @@ deps =
 
 [testenv:checkqa]
 extras = qa
+ignore_errors = true
 commands =
     flake8 --version
     flake8 --show-source --statistics {posargs:covimerage tests}
+    isort --check --diff {posargs:covimerage tests}