I can reproduce this problem when use double quotes as both outer and internal quotes.

$ echo -n '{"A":"a"}' | python3 -c "import sys,json; j=json.load(sys.stdin); print(j["A"])"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
NameError: name 'A' is not defined

This has not relation to Python, this is how the quoting in the shell works. Perhaps something in your configuration makes single quotes be interpreted as double quotes.