[otbn,python] Wrap code to 79 columns

Our Python style guide claims that we wrap code at 79 columns but our
flake8 configuration is set to allow 100 (presumably because it's too
much work to squeeze existing code into shape!).

I wrote the OTBN Python code intending it to stay at 79. Put in a
local .flake8 file to set the expected width correctly and then (lots
of lines...) re-format comments and break strings to get everything
sorted.

This stricter formatting won't be checked by CI (which runs at
top-level, so won't see the new .flake8 config file) but it *will* be
checked by people's text editors if they've got the usual Python
setup.

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/hw/ip/otbn/util/yaml_to_doc.py b/hw/ip/otbn/util/yaml_to_doc.py
index 6ef8bd3..860706e 100755
--- a/hw/ip/otbn/util/yaml_to_doc.py
+++ b/hw/ip/otbn/util/yaml_to_doc.py
@@ -269,7 +269,8 @@
 
     # If this came from the RV32I instruction set, say so.
     if insn.rv32i:
-        parts.append('This instruction is defined in the RV32I instruction set.\n\n')
+        parts.append('This instruction is defined in the '
+                     'RV32I instruction set.\n\n')
 
     # A list of errors that the instruction might cause.
     if insn.errs is not None: