Relax dependency version requirements (#167)

* Relax dependency constraints

* Bump version to `6.0.1`

* pin backoff and simplejson to major version

* Don't allow older versions

* Update changelog

* Pin minumum and major versions

CHANGELOG.md

@@ -1,7 +1,10 @@
 # Changelog
 
+## 6.0.1
+  * Pin backoff and simplejson to any version greater than or equal to the previously allowed version, up to the next major version [#167](https://github.com/singer-io/singer-python/pull/167)
+
 ## 6.0.0
-  * Bump backoff version to 2.2.1. This version drops support for python 3.5, but adds it for 3.1o [#165](https://github.com/singer-io/singer-python/pull/165)
+  * Bump backoff version to 2.2.1. This version drops support for python 3.5, but adds it for 3.10 [#165](https://github.com/singer-io/singer-python/pull/165)
 
 ## 5.13.0
   * Add support for dev mode argument parsing [#158](https://github.com/singer-io/singer-python/pull/158)

setup.py

@@ -4,18 +4,18 @@
 import subprocess
 
 setup(name="singer-python",
-      version='6.0.0',
+      version='6.0.1',
       description="Singer.io utility library",
       author="Stitch",
       classifiers=['Programming Language :: Python :: 3 :: Only'],
       url="http://singer.io",
       install_requires=[
           'pytz>=2018.4',
-          'jsonschema==2.6.0',
-          'simplejson==3.11.1',
-          'python-dateutil>=2.6.0',
-          'backoff==2.2.1',
-	  'ciso8601',
+          'jsonschema>=2.6.0,==2.*',
+          'simplejson>=3.13.2,==3.*',
+          'python-dateutil>=2.7.3,==2.*',
+          'backoff>=2.2.1,==2.*',
+          'ciso8601>=2.3.1,==2.*',
       ],
       extras_require={
           'dev': [