Add pre-commit support for sanity checks

Use pre-commit framework [1] to run black and flake8 before the commit.
black and flake8 are managed by the pre-commit framework and they can be
run manually by the user using `pre-commit run` command.

Fix the code base with the help of black and flake8.
Fix import statements according to PEP8 guidelines [1]
Both tools have the following settings (specified in the pre-commit
configuration file):
* line length: 120 characters
* directory to exclude: ethosu/vela/tflite/ and ethosu/vela/ethos_u55_regs

Updated README.md on how to install pre-commit and how to run sanity checks.
Pipenv files have been updated including new dependencies for pre-commit.

[1]: https://www.python.org/dev/peps/pep-0008/#imports
[2]: https://github.com/pre-commit/pre-commit

Change-Id: I304d9fffdf019d390ffa396a529c8a7c2437f63d
Signed-off-by: Diego Russo <diego.russo@arm.com>
diff --git a/ethosu/vela/extract_npu_subgraphs.py b/ethosu/vela/extract_npu_subgraphs.py
index 5b9ba8b..ab3db21 100644
--- a/ethosu/vela/extract_npu_subgraphs.py
+++ b/ethosu/vela/extract_npu_subgraphs.py
@@ -23,10 +23,11 @@
 # by NpuOp operations. Later, Vela generates command streams and compressed weight streams for the NPU subgraphs and
 # attaches them to the NpuOp. This encapsulates everything the NPU subgraph is supposed to do.
 
-from .nn_graph import Pass, PassPlacement, NpuBlockType, Subgraph
-from .operation import Operation
 import numpy as np
 
+from .nn_graph import Pass, PassPlacement, Subgraph
+from .operation import Operation, NpuBlockType
+
 
 def make_npu_call_op_pass(npu_subgraph):
     op = Operation("NpuOp", "call_" + npu_subgraph.name)