fix: Regex for getting aws_lambda_builders version

[jfuss]

Issue #, if available:
N/A

Description of changes:
I noticed that the version of Lambda Builders was the whole file. This gets tracked back to when we added

# new regex
>>> re.search(r"__version__ = \"([^'\"]+)", content).group(1)
'0.6.0'

# old regex
>>> re.search(r"__version__ = \"([^']+)\"", content).group(1)
'0.6.0"\nRPC_PROTOCOL_VERSION = "0.3'

This did not happen before because the __version__ was a string in single quotes and RPC_PROTOCOL_VERSION was double quotes. 557214b#diff-eeb75f3f96c23565c44aae048349b2a3 Our regex at that time, assumed only version was the only thing in single quotes in the file (aws_lambda_builders/__init__.py). When we updated to standardize strings to be all double quotes with black, I update this regex assuming the regex was already taking into account many things in the file. This PR fixes that assumption so that the version is only the __version__ value.

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

[c2tarun]

Curious to know why we are using regex instead of simply importing the module and read value from variable? Will that not work?

[jfuss]

Basically, importing runs the risk of side affects happening or further importing of modules that are not yet installed. There are more details here: https://packaging.python.org/guides/single-sourcing-package-version/

[c2tarun]

Cool, I do like approach 3 and 4 in that page, however that is not in scope of this CR :)