chore: deprecate Python 3.6 support (#3364)

Python 3.6 has reached end-of-life at the end of December 2021, and new
`pip` releases no longer support it.

Check for python versions at load of the `jsii` runtime for Python, and
emit a warning if Python < 3.7 is detected. Additionally, change the
`pip` build-tool requirement to be `pip ~= 22.0` if Python >= 3.7 is
used.



---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

Author: Romain Marcadier

packages/@jsii/python-runtime/requirements.txt

@@ -1,6 +1,7 @@
 black~=21.12b0
 mypy==0.812
-pip~=21.3
+pip~=21.3 ; python_version < '3.7'
+pip~=22.0 ; python_version >= '3.7'
 pytest~=6.2
 pytest-mypy~=0.9
 setuptools~=59.6

packages/@jsii/python-runtime/src/jsii/__init__.py

@@ -1,3 +1,4 @@
+import sys
 from typing import Union
 
 from .__meta__ import __version__, __jsii_runtime_version__
@@ -35,6 +36,15 @@
 stats = kernel.stats
 
 
+if sys.version_info < (3, 7):
+    from platform import python_version
+    import warnings
+
+    warnings.warn(
+        f"WARNING: You are using python release {python_version()}, which has reached end-of-life! Support for EOL Python releases may be dropped in the future. Please consider upgrading to Python >= 3.7 as soon as possible.",
+    )
+
+
 __all__ = [
     "__version__",
     "__jsii_runtime_version__",