fix(python): Read venv prompt from pyvenv.cfg (#1747)

* ✨ Add  --prompt rendering to python virtualenv

* Use pyvenv.cfg to find prompt

* Remove usage of VIRTUAL_ENV_PROMPT variable

Seeing how both venv and virtualenv set a pyvenv.cfg, this isn't needed.
Additionally pyvenv.cfg is set in earlier versions than VIRTUAL_ENV_PROMPT,
which at this moment is in the unrelased python 3.10

* Smarter result unwrapping thanks to clippy

Cargo.lock

@@ -1,5 +1,11 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+[[package]]
+name = "ahash"
+version = "0.4.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0adac150c2dd5a9c864d054e07bda5e6bc010cd10036ea5f17e82a2f5867f735"
+
 [[package]]
 name = "aho-corasick"
 version = "0.7.13"
@@ -333,6 +339,15 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "dlv-list"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1b391911b9a786312a10cb9d2b3d0735adfd5a8113eb3648de26a75e91b0826c"
+dependencies = [
+ "rand",
+]
+
 [[package]]
 name = "doc-comment"
 version = "0.3.3"
@@ -430,6 +445,15 @@ dependencies = [
  "autocfg",
 ]
 
+[[package]]
+name = "hashbrown"
+version = "0.9.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d7afe4a420e3fe79967a00898cc1f4db7c8a49a9333a29f8a4bd76a253d5cd04"
+dependencies = [
+ "ahash",
+]
+
 [[package]]
 name = "hermit-abi"
 version = "0.1.15"
@@ -468,7 +492,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "86b45e59b16c76b11bf9738fd5d38879d3bd28ad292d7b313608becb17ae2df9"
 dependencies = [
  "autocfg",
- "hashbrown",
+ "hashbrown 0.8.2",
 ]
 
 [[package]]
@@ -787,6 +811,16 @@ dependencies = [
  "vcpkg",
 ]
 
+[[package]]
+name = "ordered-multimap"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "87a6df8506c2d359d5105344891f283e8e7ef81a7c6a542d516f8707e4a09b6b"
+dependencies = [
+ "dlv-list",
+ "hashbrown 0.9.1",
+]
+
 [[package]]
 name = "os_info"
 version = "3.0.0"
@@ -1020,6 +1054,16 @@ dependencies = [
  "crossbeam-utils",
 ]
 
+[[package]]
+name = "rust-ini"
+version = "0.16.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dcdbf249a931ceb503aa80e08ca016675dbc7884eda8e2d88e85533d4dbade62"
+dependencies = [
+ "cfg-if",
+ "ordered-multimap",
+]
+
 [[package]]
 name = "ryu"
 version = "1.0.5"
@@ -1153,6 +1197,7 @@ dependencies = [
  "rand",
  "rayon",
  "regex",
+ "rust-ini",
  "serde_json",
  "shell-words",
  "starship_module_config_derive",