Drop python 3.7 in code-base (#11029)

Starting `22.02` we dropped support for python `3.7`: https://docs.rapids.ai/notices/rsn0014/, but there are places in code-base that still have 3.7 usages. This PR removes and upgrades those usages.

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Ashwin Srinath (https://github.com/shwina)
  - Bradley Dice (https://github.com/bdice)
  - https://github.com/jakirkham
  - AJ Schmidt (https://github.com/ajschmidt8)

URL: #11029

README.md

@@ -65,30 +65,30 @@ Please see the [Demo Docker Repository](https://hub.docker.com/r/rapidsai/rapids
 
 cuDF can be installed with conda ([miniconda](https://conda.io/miniconda.html), or the full [Anaconda distribution](https://www.anaconda.com/download)) from the `rapidsai` channel:
 
-For `cudf version == 21.08` :
+For `cudf version == 22.06` :
 ```bash
 # for CUDA 11.0
 conda install -c rapidsai -c nvidia -c numba -c conda-forge \
-    cudf=21.08 python=3.7 cudatoolkit=11.0
+    cudf=21.08 python=3.9 cudatoolkit=11.0
 
 # or, for CUDA 11.2
 conda install -c rapidsai -c nvidia -c numba -c conda-forge \
-    cudf=21.08 python=3.7 cudatoolkit=11.2
+    cudf=21.08 python=3.9 cudatoolkit=11.2
 
 ```
 
 For the nightly version of `cudf` :
 ```bash
 # for CUDA 11.0
 conda install -c rapidsai-nightly -c nvidia -c numba -c conda-forge \
-    cudf python=3.7 cudatoolkit=11.0
+    cudf python=3.9 cudatoolkit=11.0
 
 # or, for CUDA 11.2
 conda install -c rapidsai-nightly -c nvidia -c numba -c conda-forge \
-    cudf python=3.7 cudatoolkit=11.2
+    cudf python=3.9 cudatoolkit=11.2
 ```
 
-Note: cuDF is supported only on Linux, and with Python versions 3.7 and later.
+Note: cuDF is supported only on Linux, and with Python versions 3.8 and later.
 
 See the [Get RAPIDS version picker](https://rapids.ai/start.html) for more OS and version info. 
 

conda/environments/cudf_dev_cuda11.5.yml

@@ -17,7 +17,7 @@ dependencies:
   - cmake>=3.20.1,!=3.23.0
   - cmake_setuptools>=0.1.3
   - scikit-build>=0.13.1
-  - python>=3.7,<3.9
+  - python>=3.8,<3.10
   - numba>=0.54
   - numpy
   - pandas>=1.0,<1.5.0dev0

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.black]
 line-length = 79
-target-version = ["py36"]
+target-version = ["py38"]
 include = '\.py?$'
 exclude = '''
 /(

python/cudf/pyproject.toml

@@ -13,7 +13,7 @@ requires = [
 
 [tool.black]
 line-length = 79
-target-version = ["py36"]
+target-version = ["py38"]
 include = '\.py?$'
 exclude = '''
 /(

python/cudf_kafka/pyproject.toml

@@ -10,7 +10,7 @@ requires = [
 
 [tool.black]
 line-length = 79
-target-version = ["py36"]
+target-version = ["py38"]
 include = '\.py?$'
 exclude = '''
 /(

python/custreamz/pyproject.toml

@@ -9,7 +9,7 @@ requires = [
 
 [tool.black]
 line-length = 79
-target-version = ["py36"]
+target-version = ["py38"]
 include = '\.py?$'
 exclude = '''
 /(

python/dask_cudf/pyproject.toml

@@ -9,7 +9,7 @@ requires = [
 
 [tool.black]
 line-length = 79
-target-version = ["py36"]
+target-version = ["py38"]
 include = '\.py?$'
 exclude = '''
 /(