Drop support for Python 3.6

Protobuf 3.20 does not support Python < 3.7, as described in protocolbuffers/protobuf#9480

setup.py

@@ -26,7 +26,7 @@ def get_version(rel_path):
     name="ledgerwallet",
     version=get_version("ledgerwallet/__init__.py"),
     url="https://github.com/LedgerHQ/ledgerctl/",
-    python_requires=">=3.6",
+    python_requires=">=3.7",
     license="MIT",
     description="Python client and library to communicate with Ledger devices",
     long_description=long_description,
@@ -49,7 +49,6 @@ def get_version(rel_path):
     py_modules=["ledgerctl"],
     classifiers=[
         "License :: OSI Approved :: MIT License",
-        "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",