Restore python2 compatibility of EC2 plugin

[yonash2]

Description of changes:
In this PR, python2 compatibility broke.

from urllib.request import urlopen, Request will not work unless install_aliases() is called before:

❯ python2
Python 2.7.18 (v2.7.18:8d21aa21f2, Apr 19 2020, 20:48:48)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> import json
>>> import logging
>>> from future.standard_library import install_aliases
>>> from urllib.request import urlopen, Request
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named request
>>> install_aliases()
>>> from urllib.request import urlopen, Request
>>>

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #249 (c1d05bf) into master (de3ce93) will decrease coverage by 1.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
- Coverage   79.36%   78.18%   -1.18%     
==========================================
  Files          80       74       -6     
  Lines        3159     2838     -321     
==========================================
- Hits         2507     2219     -288     
+ Misses        652      619      -33     

Impacted Files	Coverage Δ
aws_xray_sdk/core/plugins/ec2_plugin.py	95.45% <100.00%> (ø)
aws_xray_sdk/ext/aiobotocore/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
aws_xray_sdk/core/__init__.py	75.00% <0.00%> (-25.00%)	⬇️
aws_xray_sdk/core/utils/compat.py	85.71% <0.00%> (-9.53%)	⬇️
aws_xray_sdk/core/patcher.py	83.20% <0.00%> (-4.00%)	⬇️
aws_xray_sdk/ext/httplib/patch.py	72.63% <0.00%> (-3.16%)	⬇️
aws_xray_sdk/core/sampling/connector.py	47.94% <0.00%> (-2.71%)	⬇️
aws_xray_sdk/core/emitters/udp_emitter.py	61.11% <0.00%> (-2.05%)	⬇️
aws_xray_sdk/ext/util.py	89.83% <0.00%> (-1.70%)	⬇️
aws_xray_sdk/core/sampling/reservoir.py	57.69% <0.00%> (-1.57%)	⬇️
... and 23 more

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[srprash]

Thanks!