Fgpyo

What's Changed
* Fix README links to the main branch by nh13 in https://github.com/fulcrumgenomics/fgpyo/pull/6
* Add classes for representing read structures by nh13 in https://github.com/fulcrumgenomics/fgpyo/pull/7


**Full Changelog**: https://github.com/fulcrumgenomics/fgpyo/compare/0.0.3...0.0.4

This is a minor release of `fgpyo`.

Bugfixes:
Various `Metric` class bugfixes:
- fixed issue with parsing `Optional` types on `attr` classes
- fixed issue with imports / naming that was being ignored due to error handling
- fixed issue with partial functions not being called correctly

Features:
- improved error handling by adding custom error class

This is a minor release of `fgpyo` to loosen the python version requirement from `>=py3.6.10` to `>=3.6.0` (1)

This is the first release of `fgpyo` version 0.0.1.

Modules included are:
* `fgpyo.sam.SupplementaryAlignment`: stores a supplementary alignment record produced by BWA and stored in the SA SAM tag
* `fgypo.sam.Metric`: a class for all metric-like tab-delimited files. Metric files are tab-delimited, contain a header, and zero or more rows for metric values. This makes it easy for them to be read in languages like `R`. This allows implementing classes to extend [`attr`](https://www.attrs.org/en/stable/examples.html) classes, have attributes be _typed_, and read and write to tab-delimited files with types properly preserved. This is inspired from the [`Metric` in the fgbio-scala repository](https://github.com/fulcrumgenomics/fgbio/blob/master/src/main/scala/com/fulcrumgenomics/util/Metric.scala).
* ancillary methods to support inspecting and maintaining types in `fgypo.sam.Metric`