Only check_uppercase for string genes

[brenshanny]

This fixes a bug where parse_genelists converts the gene list into a set of ints which then throws an exception when calling isupper() on an int

/usr/local/lib/python3.11/dist-packages/gseapy/__init__.py:554: in enrichr
    enr.run()
/usr/local/lib/python3.11/dist-packages/gseapy/enrichr.py:612: in run
    self._gene_isupper = self.check_uppercase(self._gls)
/usr/local/lib/python3.11/dist-packages/gseapy/enrichr.py:266: in check_uppercase
    is_upper = [s.isupper() for s in gene_list]
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

.0 = <set_iterator object at 0x7fe68eac7300>

>   is_upper = [s.isupper() for s in gene_list]
E   AttributeError: 'int' object has no attribute 'isupper'