Bump up the minimum required version of ply (#5890)

Avoid the test failure in `sycamore_gate_test.py` such as follows:

```
$ pytest -x -q cirq-google/cirq_google/ops/sycamore_gate_test.py::test_consistent_protocols
...
        if not optimize:
            if linfo.validate_all():
>               raise SyntaxError("Can't build lexer")
E               SyntaxError: Can't build lexer

/tmp/t39/lib/python3.9/site-packages/ply/lex.py:894: SyntaxError
-------------------------------------------- Captured stderr call --------------------------------------------
ERROR: /tmp/t39/lib/python3.9/site-packages/qiskit/qasm/qasmlexer.py:191: No regular expression defined for rule 't_eof'
```

cirq-core/cirq/contrib/requirements.txt

@@ -1,6 +1,6 @@
 # Runtime requirements for contrib.
 
-ply>=3.4
+ply>=3.6
 pylatex~=1.3.0
 
 # quimb