Add entry point for python3 command

Issue:

On ubuntu, there is an entry point for both `python3` and `python3`.

On windows, `python3` doesn't exist.

Not sure about osx.

Could windows be added an entry point for the command `python3` so that we can reach a broader audiance with "fool proof" instructions.

Ubuntu

Windows: (I deleted things in my roaming path between the two commands)

Environment (conda list):

```
$ conda list
# packages in environment at C:\Users\mark\miniconda3:
#
# Name Version Build Channel
anaconda-client 1.7.2 py_0 conda-forge
attrs 19.3.0 py_0 conda-forge
beautifulsoup4 4.9.1 py37hc8dfbb8_0 conda-forge
blinker 1.4 py_1 conda-forge
boolean.py 3.7 py_0 conda-forge
brotlipy 0.7.0 py37h4ab8f01_1000 conda-forge
bzip2 1.0.8 hfa6e2cd_2 conda-forge
ca-certificates 2020.4.5.1 hecc5488_0 conda-forge
certifi 2020.4.5.1 py37hc8dfbb8_0 conda-forge
cffi 1.14.0 py37ha419a9e_0 conda-forge
chardet 3.0.4 py37hc8dfbb8_1006 conda-forge
clyent 1.2.2 py_1 conda-forge
conda 4.8.3 py37hc8dfbb8_1 conda-forge
conda-build 3.19.2 py37hc8dfbb8_2 conda-forge
conda-forge-pinning 2020.05.25.23.01.15 0 conda-forge
conda-package-handling 1.6.0 py37h702c6c1_2 conda-forge
conda-smithy 3.7.3 py_0 conda-forge
console_shortcut 0.1.1 4
cryptography 2.9.2 py37h26f1ce3_0 conda-forge
decorator 4.4.2 py_0 conda-forge
deprecated 1.2.10 pyh9f0ad1d_0 conda-forge
filelock 3.0.12 pyh9f0ad1d_0 conda-forge
git 2.26.2 0 conda-forge
gitdb 4.0.5 py_0 conda-forge
gitpython 3.1.2 py_0 conda-forge
glob2 0.7 py_0 conda-forge
idna 2.9 py_1 conda-forge
importlib-metadata 1.6.0 py37hc8dfbb8_0 conda-forge
importlib_metadata 1.6.0 0 conda-forge
ipython_genutils 0.2.0 py_1 conda-forge
isodate 0.6.0 py_1 conda-forge
jinja2 2.11.2 pyh9f0ad1d_0 conda-forge
jsonschema 3.2.0 py37hc8dfbb8_1 conda-forge
jupyter_core 4.6.3 py37hc8dfbb8_1 conda-forge
krb5 1.17.1 hdd46e55_0 conda-forge
libarchive 3.3.3 h0c0e0cf_1008 conda-forge
libcurl 7.69.1 h1dcc11c_0 conda-forge
libiconv 1.15 hfa6e2cd_1006 conda-forge
liblief 0.10.1 ha925a31_0
libsolv 0.7.13 h7bd577a_3 conda-forge
libssh2 1.9.0 h3235a2c_2 conda-forge
libxml2 2.9.10 h5d81f1c_1 conda-forge
license-expression 1.2 py_0 conda-forge
lz4-c 1.9.2 h62dcd97_1 conda-forge
lzo 2.10 hfa6e2cd_1000 conda-forge
m2-msys2-runtime 2.5.0.17080.65c939c 3
m2-patch 2.7.5 2
m2w64-gcc-libgfortran 5.3.0 6
m2w64-gcc-libs 5.3.0 7
m2w64-gcc-libs-core 5.3.0 7
m2w64-gmp 6.1.0 2
m2w64-libwinpthread-git 5.0.0.4634.697f757 2
mamba 0.2.12 py37h9da7104_0 conda-forge
markupsafe 1.1.1 py37h8055547_1 conda-forge
menuinst 1.4.16 py37_0 conda-forge
msrest 0.6.10 py_0 conda-forge
msys2-conda-epoch 20160418 1
nbformat 5.0.6 py_0 conda-forge
oauthlib 3.0.1 py_0 conda-forge
openssl 1.1.1g he774522_0 conda-forge
pip 20.1.1 pyh9f0ad1d_0 conda-forge
pkginfo 1.5.0.1 py_0 conda-forge
powershell_shortcut 0.0.1 3
psutil 5.7.0 py37h8055547_1 conda-forge
py-lief 0.10.1 py37ha925a31_0
pycosat 0.6.3 py37h8055547_1004 conda-forge
pycparser 2.20 py_0 conda-forge
pycrypto 2.6.1 py37h8055547_1004 conda-forge
pygithub 1.51 py_0 conda-forge
pyjwt 1.7.1 py_0 conda-forge
pyopenssl 19.1.0 py_1 conda-forge
pyrsistent 0.16.0 py37h8055547_0 conda-forge
pysocks 1.7.1 py37hc8dfbb8_1 conda-forge
python 3.7.6 h60c2a47_5_cpython conda-forge
python-dateutil 2.8.1 py_0 conda-forge
python-libarchive-c 2.9 py37_0 conda-forge
python_abi 3.7 1_cp37m conda-forge
pytz 2020.1 pyh9f0ad1d_0 conda-forge
pywin32 227 py37hfa6e2cd_0 conda-forge
pyyaml 5.3.1 py37h8055547_0 conda-forge
requests 2.23.0 pyh8c360ce_2 conda-forge
requests-oauthlib 1.2.0 py_0 conda-forge
ripgrep 12.1.0 h301d43c_0 conda-forge
ruamel.yaml 0.16.6 py37h8055547_1 conda-forge
ruamel.yaml.clib 0.2.0 py37h8055547_1 conda-forge
ruamel_yaml 0.15.80 py37h8055547_1001 conda-forge
scrypt 0.8.15 py37h90766aa_0 conda-forge
setuptools 46.4.0 py37hc8dfbb8_0 conda-forge
six 1.15.0 pyh9f0ad1d_0 conda-forge
smmap 3.0.4 pyh9f0ad1d_0 conda-forge
soupsieve 2.0.1 py37hc8dfbb8_0 conda-forge
sqlite 3.30.1 hfa6e2cd_0 conda-forge
tk 8.6.10 hfa6e2cd_0 conda-forge
toolz 0.10.0 py_0 conda-forge
tqdm 4.46.0 pyh9f0ad1d_0 conda-forge
traitlets 4.3.3 py37hc8dfbb8_1 conda-forge
urllib3 1.25.9 py_0 conda-forge
vc 14.1 h869be7e_1 conda-forge
vs2015_runtime 14.16.27012 h30e32a0_2 conda-forge
vsts-python-api 0.1.22 py_0 conda-forge
wheel 0.34.2 py_1 conda-forge
win_inet_pton 1.1.0 py37_0 conda-forge
wincertstore 0.2 py37_1003 conda-forge
wrapt 1.12.1 py37h8055547_1 conda-forge
xz 5.2.5 h2fa13f4_0 conda-forge
yaml 0.2.4 he774522_0 conda-forge
zipp 3.1.0 py_0 conda-forge
zlib 1.2.11 h2fa13f4_1006 conda-forge
zstd 1.4.4 h9f78265_3 conda-forge
```

Details about conda and system ( conda info ):

```
(base) C:\Users\mark>conda info

active environment : base
active env location : C:\Users\mark\miniconda3
shell level : 1
user config file : C:\Users\mark\.condarc
populated config files : C:\Users\mark\.condarc
conda version : 4.8.3
conda-build version : 3.19.2
python version : 3.7.6.final.0
virtual packages : __cuda=10.1
base environment : C:\Users\mark\miniconda3 (writable)
channel URLs : https://conda.anaconda.org/ramonaoptics/win-64
https://conda.anaconda.org/ramonaoptics/noarch
https://conda.anaconda.org/conda-forge/win-64
https://conda.anaconda.org/conda-forge/noarch
https://repo.anaconda.com/pkgs/main/win-64
https://repo.anaconda.com/pkgs/main/noarch
https://repo.anaconda.com/pkgs/r/win-64
https://repo.anaconda.com/pkgs/r/noarch
https://repo.anaconda.com/pkgs/msys2/win-64
https://repo.anaconda.com/pkgs/msys2/noarch
package cache : C:\Users\mark\miniconda3\pkgs
C:\Users\mark\.conda\pkgs
C:\Users\mark\AppData\Local\conda\conda\pkgs
envs directories : C:\Users\mark\miniconda3\envs
C:\Users\mark\.conda\envs
C:\Users\mark\AppData\Local\conda\conda\envs
platform : win-64
user-agent : conda/4.8.3 requests/2.23.0 CPython/3.7.6 Windows/10 Windows/10.0.18362
administrator : False
netrc file : None
offline mode : False
```