Upgrade to Pip 21.1. (#3070)

Upgrade to Pip 21.1 in preparation to upgrade DBT 19.

The latest Pip has

- a stricter resolver
- access to more up to date Python wheels (e.g. #2267 is solved since as any version after Pip 21 has access to ready-compiled Python wheels, see https://issues.apache.org/jira/browse/ARROW-11835).
- in theory faster pip installs (pypa/pip#9187 (comment) makes me think the current Pip version is stable enough be consumed)

Also relax the constraint for `requests` and `backoff` since both were actually specifying incompatible versions that the old Pip resolver was silently swallowing.

Test this locally first by running git clean -dfx to remove all the existing venv files. The two different Pip versions do not work well.

airbyte-integrations/bases/base-python/setup.py

@@ -36,8 +36,8 @@
         "pydantic==1.6.1",
         "airbyte-protocol",
         "jsonschema==2.6.0",
-        "requests==2.25.1",
-        "backoff==1.10.0",
+        "requests",
+        "backoff",
         "pytest",
         "pendulum",
     ],

buildSrc/src/main/groovy/airbyte-python.gradle

@@ -23,7 +23,7 @@ class AirbytePythonPlugin implements Plugin<Project> {
             pip 'mypy:0.790'
             pip 'isort:5.6.4'
             pip 'pytest:6.1.2'
-            pip 'pip:20.2'
+            pip 'pip:21.1'
         }
 
         project.task('isortFormat', type: PythonTask) {