Bump dev requirements

Also re-order lexicographically.

Note that I did not exhaustively test this... there could be edge cases
depending on the python version. But I think we should be okay because
`tox.ini` is currently testing using with unpinned versions, so I think
we're already running these versions in our test suite.

requirements-dev.txt

@@ -1,17 +1,17 @@
-flake8==3.4.1
-pytest==5.4.3
-pytest-cov==2.10.0
+coveralls==2.1.2
+crc32c==2.1
 docker-py==1.10.6
-coveralls==1.5.1
-Sphinx==1.6.4
-lz4==2.1.2
-xxhash==1.3.0
-python-snappy==0.5.3
-tox==3.5.3
-mock==3.0.5
-pylint==2.5.3
+flake8==3.8.3
+lz4==3.1.0
+mock==4.0.2
+py==1.9.0
+pylint==2.6.0
+pytest==6.0.2
+pytest-cov==2.10.1
+pytest-mock==3.3.1
 pytest-pylint==0.17.0
-pytest-mock==1.10.0
-sphinx-rtd-theme==0.2.4
-crc32c==2.1
-py==1.8.0
+python-snappy==0.5.4
+Sphinx==3.2.1
+sphinx-rtd-theme==0.5.0
+tox==3.20.0
+xxhash==2.0.0