fix python3 run_time_error in uniform_random. test=develop

[fc-liu]

fix runtimeerror : dictionary changed size during iteration when calling uniform_random in python3+
related #17086

[junjun315]

LGTM

[luotao1]

python35 has a random fail unit-test, I rerun it and please wait.

[luotao1]

$ grep -rn "locals_var = locals().keys()" python/paddle/fluid/layers/ops.py
python/paddle/fluid/layers/ops.py:105:    locals_var = locals().keys()
python/paddle/fluid/layers/ops.py:127:    locals_var = locals().keys()
python/paddle/fluid/layers/ops.py:149:    locals_var = locals().keys()

Could you update below three lines in a new PR? Thanks very much for your contribution!
@Lfc1993