Drop support for Python 3.6

[acezen]

What do these changes do?

since the NetworkX dependency has been bump up to 2.6 and it has droped support for Python3.6, the GraphScope should drop support for python 3.6.

Related issue number

Fixes #1269

[codecov-commenter]

Codecov Report

Merging #1268 (0d089dd) into main (d3e8545) will decrease coverage by 33.37%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main    #1268       +/-   ##
===========================================
- Coverage   73.70%   40.32%   -33.38%     
===========================================
  Files          89       89               
  Lines        9476     9476               
===========================================
- Hits         6984     3821     -3163     
- Misses       2492     5655     +3163     

Impacted Files	Coverage Δ
python/graphscope/tests/unittest/test_lazy.py	0.00% <0.00%> (-100.00%)	⬇️
python/graphscope/tests/unittest/test_java_app.py	0.00% <0.00%> (-100.00%)	⬇️
...ython/graphscope/tests/unittest/test_cython_ast.py	0.00% <0.00%> (-100.00%)	⬇️
...ython/graphscope/tests/unittest/test_serailaize.py	0.00% <0.00%> (-100.00%)	⬇️
python/graphscope/tests/unittest/test_app.py	0.00% <0.00%> (-98.39%)	⬇️
python/graphscope/analytical/udf/patch.py	3.47% <0.00%> (-96.53%)	⬇️
...thon/graphscope/tests/unittest/test_scalability.py	0.00% <0.00%> (-96.16%)	⬇️
...hon/graphscope/tests/unittest/test_create_graph.py	0.00% <0.00%> (-92.00%)	⬇️
python/graphscope/tests/unittest/test_graph.py	0.00% <0.00%> (-85.22%)	⬇️
python/graphscope/tests/unittest/test_context.py	0.00% <0.00%> (-81.04%)	⬇️
... and 36 more

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