Deprecate Python 3.6 support, v0.14 last supported version (#751)

Implement deprecation warning in root __init__.
A warnings filter, specifically for this deprecation warning,
has been prepended to ensure it will be shown once,
when anything is imported from the `optimade` package.

Author: CasperWA

optimade/__init__.py

@@ -1,2 +1,21 @@
 __version__ = "0.13.3"
 __api_version__ = "1.0.1"
+
+
+import sys
+
+if sys.version_info.minor == 6:
+    # Python 3.6
+    import warnings
+
+    warnings.filterwarnings(
+        action="once",
+        message=r"v0\.14 of the `optimade` package.*",
+        category=DeprecationWarning,
+        append=False,
+    )
+    warnings.warn(
+        "v0.14 of the `optimade` package will be the last to support Python 3.6. "
+        "Please upgrade to Python 3.7+ to use v0.15 and later versions of `optimade`.",
+        DeprecationWarning,
+    )