[BUG] cuDF expression evaluation of some comparison operators creates AssertionError

[karthikeyann]

Describe the bug
cuDF expression evaluation created AssertionError for less equal, greater equal and not equal operators.

Steps/Code to reproduce bug

import cudf
df = cudf.DataFrame({'a': 1, 'b': 's'})
df.to_pandas().eval("a>=2")
df.to_pandas().eval("a<=2")
df.to_pandas().eval("a!=2")

#these statements error ou
df.eval("a>=2")
df.eval("a<=2")
df.eval("a!=2")

[vyasr]

The problem is this line of code that is using a regex to search for an assignment operator. It correctly filters out ==, but is not currently aware of >= or <=. Adding those to characters to the excluded character classes should fix the issue.