make: Auto-format Python files, and update existing

This patch adds extends `make fmt` to auto-format Python files using
`black`, and also runs it on the existing files.

internal/tlsconst/generate-ciphers.py

@@ -38,8 +38,7 @@ def getCiphers():
 ciphers = getCiphers()
 
 out = open(sys.argv[1], "w")
-out.write(
-    """\
+out.write("""\
 package tlsconst
 
 // AUTOGENERATED - DO NOT EDIT
@@ -47,8 +46,7 @@ package tlsconst
 // This file was autogenerated by generate-ciphers.py.
 
 var cipherSuiteName = map[uint16]string{
-"""
-)
+""")
 
 for ver, desc in ciphers:
     out.write('\t%s: "%s",\n' % (ver, desc))