move __version__ and user_agent to library-level constants to improve performance

[jreiberkyle]

Consider raising _get_user_agent() to a class or module-level constant instead of a non-public static method for UX as well as performance.

Originally posted by @ericrdunham in #240 (comment)

Jen comments: I like this suggestion. I don't like having to update the user agent to stay up to date with the version. I am not sure I really like the pattern of having a module called __version__.py that basically holds a constant, __version__ = 'blah'.

Possible implementation

add lines to constants.py:

VERSION = 'blah'
USER_AGENT = 'planet_client_python/' + VERSION

[jreiberkyle]

All the examples of clients I’m seeing are either specifying version in __version__.py or __init__.py

I’m thinking this is because one needs to be able to get the version from the library with

import planet
planet.__version__

example of user_agent being specified as a module-level constant: https://github.com/encode/httpx/blob/master/httpx/_client.py#L67

[jreiberkyle]

result from team discussion: have __version__ specified in either __init__.py or __version__.py

refs:
https://www.python.org/dev/peps/pep-0008/#module-level-dunder-names
https://packaging.python.org/guides/single-sourcing-package-version/#single-sourcing-the-version